There is so much that is wrong with the United Nations in the practical sense. The corruption, the support and protection the organisation grants to dictators, the deflection of criticism of genocide are only a few of these problems.
But there is a more fundemantal problem with the UN which is probably the cause for all the practical problems, and that problem is with how the UN completely ignore their own purpose.
When the United Nations were founded during World War II by the later victors of the same conflict, the ideals were noble and the concept was sound. But within ten years the United Nations started ignoring those principles.
Here are some procedural issues with the "United Nations" of the last few decades:
1. Not all nations are represented.
And neither is there a mechanism to allow non-represented nations to gain representation.
In fact the UN provide many mechanisms to member states specifically to deny representation of nations ruled by member states.
2. The United Nations do not define "nation" according to any useful definition.
Instead of the usual definitions based on common history, language, religion or ethnic origin, the UN simply accepts whomever manages to gain power in a certain region (defined by the UN) as the representative for a "nation" made up on the spot by that individual.
3. Of the represented nations some have one vote and some have more than one vote.
This appears to have nothing to do with the size of the nations.
4. All nations are really represented by states.
This is in effect a mechanism to ensure that not all nations can gain representation since many states rule over many nations.
The Welsh nation is in the UN represented by the United Kingdom, as are the English nation, the Scottish nation, and one third of the Irish nation. (The other two thirds of the Irish nation are represented by the Republic of Ireland.)
5. Member states are not all of the same type.
Some UN members are democracies, others are responsible absolute monarchies, many are simply dictatorships with no respect for human rights. All of those are treated as "equals" by the UN. But those that have more support among other dictatorships are even more equal.
UN member states neither always represent nations nor do they even have a mechanism for representing anyone except their own governments.
The Vatican has observer status in the UN. Which nation does the Vatican represent? The Pope? Doesn't a nation at least require one male and one female member to function?
How can this be reformed? I have no idea. There is perhaps no better mechanism. But on the other hand, which problem is the UN solving anyway?
What could be done is the United "Nations" could give up their claim to represent "nations" and make it clear that they merely represent governments. Where a government is actually elected, the government could claim to represent a state, and where that state is defined as a nation state, the state could claim to represent that nation.
But the dictatorial ruler of a random African country covering areas inhabited by several nations cannot claim to represent a "nation" made up of the population he controls for the same reason that a raving mad man taking control of an apartment building cannot claim to be the father of a "family" made up of the people in that apartment building.
A family association made up not of families but of unmarried weirdos elected to represent apartment buildings (or keeping people of such a building as hostages) would very likely not be respected as an organisation representing "families", at least not if more than half of its single male members control their blocks using guns only.