THE big thing, as the United Nations embarks on its 50th General Assembly with a world summit meeting, social galas, and orotund pronouncements, is UN reform. The need is clear. The UN Charter, written in 1945, today applies to a different world, without the adaptability that keeps the US Constitution forever up to date. Alas, and unavoidably, the reform process is in the hands of a committee - the membership. The United States is the most fervent advocate of drastic revision, cracking the whip for efficient administration and strait-laced finances. Yet, on Oct. 1, the US will, like it or not, cut its assessed contribution for peacekeeping operations by one-fifth, from 31.4 percent to 25 percent. No matter that the US was part of the assessment process and agreed, perhaps unwisely, to the scale of payments. Congress thinks it is not getting its money's worth. At the same time, the United States does not challenge the 25 percent that it must pay for the regular UN budget as the largest contributor. On the contrary, it insists on maintaining that level. Secretary-General Boutros Boutros-Ghali last month wrote, ''The organization's excessive dependence on the assessments from a single member state is unhealthy.'' He urged ''a more even distribution of assessments among countries with the capacity to pay'' (some have proposed setting a maximum of 10 percent for any one member). Washington's objection to that could have been heard in New York without a telephone. It wants to be the 800-pound gorilla, which might be morally tolerable if the United States actually paid its dues. It is in arrears, as of Aug. 15, by over $1 billion; certainly not the only deadbeat but far and away the largest. Furthermore, the Republican Congress, with the budget knife in its teeth, keeps threatening further withholding if it does not get its way on reform. And the administration, while protesting, seems in no mood to fight it. Speak of Pecksniff. It is well known that the UN, having had functions and functionaries plastered onto it for 50 years, needs to be trimmed down, its duplications cut out, red tape barbered, and patronage reduced. But the US, while acting as the censorious proprietor, is part of the problem. Some of its impulses stand in the way of serious internal efforts at reform. One of its major demands was appointment of an inspector-general to root out waste and corruption, informing governments as he went along. Congress threatens to withhold 50 percent of the US contribution if his work is not satisfactory. He is also to review programs, make mandatory recommendations to all UN agencies, and be a sort of UN czar, which utterly contravenes the UN Charter. A respected German official, Karl Theodor Paschke, placed in the job, finds the US breathing on his neck for dramatic results, in the absence of which Washington calls him inadequate. How the UN secretariat operates is one object of reform. Another is how the 185 equal members of the General Assembly and the, at present, 15 unequal members of the Security Council decide what the UN should do. Congress acts as though the UN were somehow a hostile, alien body in which this country is at a disadvantage - a politically profitable appeal to the xenophobic, isolationist streak in public opinion that surfaces in stressful times. No matter that it is quite untrue. The UN has never forced the US to do something against its will or kept it from acting as it felt it had to. In the Security Council, the US and the other four permanent members have veto power. This is a source of complaint since it has been misused so often in the past for matters not of vital national interest. But the suggestion that the veto be limited is a non-starter, as is a proposal that Britain and France surrender their seats to one rotating representative of the European Union. Council membership may well be expanded to be more representative of today's world. Washington favors permanent seats, without veto, for Germany and Japan and three more nonpermanent seats. It does not support proposed additional permanent seats for three major regional states such as India, Brazil, and South Africa, together with three more non-permanent third-world seats, for a total of 23. How either increase would affect Council performance in such sensitive areas as peacekeeping and peace enforcement is open to question. Incidentally, Bosnia makes it clear that peace enforcement remains an option, to be undertaken not by the UN but by a posse of member states under UN auspices. Indispensable for the reform and revival of the United Nations is American leadership, backed by a public informed about the ways the UN (an American construction, after all) can serve US interests. Disinformation is part of the problem; witness the horror story of Somalia, where the slaughter of 18 American soldiers was pinned on the UN. These men were, in fact, on an operation authorized in a Security Council resolution drafted by the US. The operation was exclusively under US command and control. When the United States has dithered, as it did in the first four years of the Yugoslav tragedy, hundreds of UN resolutions become a sorry joke. Only this summer did Washington's new determination to break the logjam suddenly dispel European fears and galvanize NATO. It could have happened in 1992. This is a cautionary tale. The UN, like any club, is no better than its members. The United States, the strongest of all, sets the tone for better or worse.