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On 16 September 2015, Nepal drafted a new constitution. https://thediplomat.com/2015/09/nepals-new-constitution-65-years-in-the-making/
Who can decide whether a constitution can be changed?
In most countries amendments to the constitution can be brought by voting in the lower house. But drafting a new constitution seems to be a different ball game together. If a party enjoys a large portion of seats in the parliament, it is possible that it may just change the constitution to benefit itself.
In the particular case of Nepal, the constitution was adopted by an elected body specifically chosen to write the constitution. This was called the Constitutional Assembly. This assembly gets its authority from the 2006 interim constitution, which was adopted following a pro-democracy movement which removed the King.
The previous constitutions had given the King greater power, but in a succession of popular revolts, he lost all authority. The Parliament claimed the right to set a new constitution. This act was fundamentally unconstitutional. But had sufficient popular support to be enacted. Ultimately any constitution is subject to the principle of "right of conquest" or "Might makes Right". A movement which is strong enough (politically, or militarily) doesn't need to worry about the law.
It took a long time, and several attempts for the Assembly to draft a constitution, but it was finally adopted in 2015. It's adoption was legal under the terms of the 2006 constitution.
AFAIK, the change of Constitutions are "affirmative actions"1; since they are the basic law of the country there is no other law that can rule if they are "legal" or not. So, their only actual basis is "because I/we say so".
Now, there are a number of methods to give Constitution a degree of legitimacy:
Referenda asking for the people to approve or reject the new Constitution.
Approval by popularly elected representatives; either through the regular institutions (i.e. the representatives elected for the legislative body) or to specifically apointed Constitutional Conventions (representatives elected only to draft the new Constitution).
Imposition from the Head of State and/or single party (in dictatorships).
Those methods are not exclusive; you could have a Constitutional Convention draft a Constitution and then present it in a referendum to the public at large; or a dictatorship can decide to allow their legislative body2 to discuss the new Constitution as a way to give it credibility and also to cater to different sensibilities in the dictatorship power base.
2After all, if it is a dictatorship the members of the legislative will be most likely in line with the Head of State.
