How can changes be made to a project management plan once the project manager baselines it?

By generating a change request and having it approved through the Perform Integrated Change Control process.

This is typically done by using a version control system. The project manager creates a new version with the changes and baselines it as the new baseline.

This is done by maintaining a revision history or log. The project manager documents the changes in detail and then baselines it.

A project management plan cannot be changed once it is baselined. An addendum has to be created to incorporate the changes.