What are the most pressing arguments for reform of the UN? How might these be achieved?

The most pressing argument is that a State is, by definition, that institution, or that group within society that claims a legal monopoly of use of aggressive force, and threats of aggressive force.

There is little enough reason to think that States will serve their subject peoples, rather than serve their own vested interests, when the States are small enough for the people to know the rulers. The bigger and more removed a State gets, and the less accountable it is to the people paying for it, the more it will become just a corrupt organisation running programs that are actively anti-social, wasteful and stupid.

The best way to reform the UN is to abolish it but short of that, it's budget should be reduced to 10 percent.

Abolishment probably is the best remedy for this strange monster that as evolved from a small monster in its beginning to an unstoppable beast today funded by strange groups with stranger still agendas. If after the world destroys itself, no small measure of that destruction will probably be pinned down on the UN which grinds down the world populations each and every day.;