The UN was ironically created because the League of Nations failed and was considered ineffective. But the UN relies almost entirely on the Security Council and the 5 permanent seats will always veto themselves. There is literally no way, for example, to suspend the US from the UN as the Security Council has to make a recommendation to the general assembly... which the US can veto as soon as the proposition is raised.
The UN is a vestige from a bygone era; the 5 permanent seats were given to the victors of World War 2 (why?) and I'm thankful we at least have China and Russia in it because imagine how much worse it would be if the permanent seats were all occupied by US satellites. You can't make any changes to the UN because if a permanent member doesn't like it they will veto it.
To dismantle the UN, it's necessary to build dual power -- a competing structure, maybe BRICS? Maybe something else, that will eventually become more interesting to the world than the UN is. And this happens through the imperialized periphery, as they represent over 80% of the world population with one thing that unites them: that of being imperialized.