UNO is an intergovernmental organization which was made to replace the ineffective League of Nations in 1945 so as to avoid another world war. But during cold war, USA and USSR often paralysed the organization and it intervened only in conflicts nit related to cold war thereby not fulfilling its mandate of peacekeeping in world. 

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Although it did help in decolonization process. Even in post-cold war era it failed in its mission as seen in Somalia and Rwanda crisis. Several organs of UNO like UNESCO and UNICEF faced charges of corruption due to which several countries withdrew their funding. So due to above issues and also due to dynamically changing polarity of world and other emerging humanitarian, environmental crisis it certainly need some reforms.

Structural reforms 

• Expansion of UNSC: The composition no longer reflects global geopolitical realities. Indeed, the Western Europe and Other Group (WEOG) now accounts for three of the five permanent members (France, the United Kingdom, and the US). That leaves only one permanent position for the Eastern European Group (Russia), one for the Asia-Pacific Group (China), and none for Africa or Latin America.
• The rotating seats on the Security Council do not adequately restore regional balance. Even with two of the ten rotating Security Council seats, the Asia-Pacific region is still massively under-represented. The Asia-Pacific region accounts for roughly 55% of the world’s population and 44% of its annual income but has just 20% (three out of 15) of the seats on the Security Council.
• Asia’s inadequate representation poses a serious threat to the UN’s legitimacy, which will only increase as the world’s most dynamic and populous region
• Do away with Veto power. 
• UN Secretariat Transparency reform: Make the UN administration (usually called the UN Secretariat or “the bureaucracy”) more transparent, more accountable, and more efficient, including direct election of the Secretary-General by the people.
• Efficiency and transparency watchdog to remove corruption issues. – Funding should not be voluntary and must be based on CBDR principle.

Functional reforms

• Decisions should be as far as possible based on consensus. 
• Organizations like NATO should be avoided and be replaced by UNO forces in conflicts.
• UN secretary general should not be a rubber stamp and need more powers.

Is UN losing its significance

• Failure to ensure World Peace (Inability to stop devastating war in Syria, The Iraq invasion, the Libyan toppling, etc. happened in violation of UN protocol, and with little consequences, North Korea) 
• Changing world order (Emergence of global south and G4 UNSC bid), not reflected in a UN based on PostWW2 geopolitics (thwarted by leading powers).
• Most of the debate revolves around the stale questions of new permanent members of the security council or restrictions on the use of the veto. Both are desirable but neither will happen soon – or fix the deeper problem.
• Alternate institutions have emerged dealing with most of UN functions (G2’s role in eco recovery, G8’s role in global politics, etc.) that make its relevance questionable.
• The basic problem is that the council’s founding premise of a world ordered by states no longer holds. Conflicts involving Boko Haram or Islamic state originate from particular local circumstances but often have regional and global reach and consequence. Designed to prevent and arbitrate state-to-state conflict, the council has failed to adapt to an entirely different world.
• New challenges have emerged (Cyber Security, Global commons, Climate Change) that the UN lacks the institutional expertise to deal with. 
• The secretariat is subjected to micro-management by member states through various committees of the general assembly

However, the need for a UN is acute as: 

• The only credible, respected and truly global organization.
• A forum to engage in debates and discussions to resolve global issues
• The only way to ensure global(near) consensus on critical issues(E.g. UNFCCC) 
• Continues to be relevant to Global security and Human rights (UNCHR role, UNPKF, etc.) 
• The only way to deal with rogue or belligerent nations, without escalation fears(Sanctions against Iran, Russia, North Korea).

Conclusion

The UN thus continues to be relevant even today, and any talks of its replacement, especially minus a viable alternative, are only immature.