Answer | Part IV of the Vienna Convention on Law of Treaties, 1969 deals with amendment and modification of treaties. It contains Article 39-41.
AMENDMENT OF TREATIES :
General Rule : A treaty may be amended by a further treaty between the parties. [Article 39]. Article 39 of the Vienna Convention of 1969 uses the words “by agreement between the parties” and not “all parties” for an amendment. Amendment of bilateral treaties does not create any problem.
The requirements of conclusion of a treaty, for example, capacity, full powers, adoption, authentication, are also necessary for the amendment of a treaty.
Amendment of Multilateral treaties : In case of multilateral treaties, such amendments, subject to a contrary provision, may require the consent of all parties to the treaty.
Articles 40 and 41 of the 1969 Convention lay down the principles governing procedure and effect of the amendment of multilateral treaties.
If the proposal is one to amend multilateral treaty between all the parties, then it must be notified to all contracting States. All contracting states have right to participate in the decision as to the action to be taken on the proposal and in the negotiation and conclusion of any amending agreement.
The amending agreement does not bind any State already a party to the treaty which does not become a party to the amending agreement. [Article 40(4)]
Any State which becomes a party to the treaty after the entry into force of the amending agreement shall, failing an expression of a different intention by that State: (a) be considered as a party to the treaty as amended; and (b) be considered as a party to the unamended treaty in relation to any party to the treaty not bound by the amending agreement. [Article 40(5)]
Article 41 deals with “Agreements to modify multilateral treaties between certain of the parties only”
Two or more of the parties to a multilateral treaty may conclude an agreement to modify the treaty as between themselves alone if: (a) the possibility of such a modification is provided for by the treaty; or (b) the modification in question is not prohibited by the treaty & it should not affect enjoyment of rights of other parties & such modification should not result in derogation of provision which would make the purpose of treaty as incompatible.