If not disbanded, then it needs to be severely reformed. Let's start with the most simple reason: It needs an army in order to do anything.
It's just a worthless figurehead. Easy to disobey, easy to abuse. War can happen without any nation receiving sanctions. Why? Because the nations that usually declare war are permanent members on the security council, and they'll obviously vote against sanctions against themselves!
Step 1 to making the UN actually worth more than dogshit: GIVE THEM AN ARMY.
Have each member on the security council give a specific amount of troops to the "United Army" (the UN's Army...I came up with the name). Permanent members must keep these troops in the United Army, temporary members may or may not take these troops out of the army once they are no longer members on the council.
Step 2 is also simple: MAKE DECISIONS MADE BY THE SECURITY COUNCIL HAVE A MAJORITY APPROVAL REQUIREMENT
Meaning, instead of needing universal approval for a simple sanction, have it be a majority rules situation. Or, at least, a "2/3 of the members need to approve" situation.
Implementing these two simple things will give the UN power to stop another Rwandan Genocide, to stop more pirate attacks, to stop the Darfur Genocide, etc. that it's supposed to take care of.