Upvote:6
It seems useful to consider the example of D&C Section 138. Section 138 was a revelation from 1918 but not added until 1981. A section from Sperry Symposium Classics: The Doctrine and Covenants from chapter 22 with the heading From Scripture to Canon describes a process which no doubt illustrates the general manner in which the leaders of the Church would undertake to update the 'LDS canon' as others may call it. The pertinent quote is:
At a meeting of the Council of the First Presidency and the Quorum of the Twelve held in the Salt Lake Temple on March 25, 1976, approval was given to add to the Pearl of Great Price...
The answer is apparently that all 15 apostles approve such a change, and we can easily assume acting on prior consideration and after seeking inspiration from God.