To unlock a master document from the master document itself, you don’t need to know from which project copy it was locked. However, you can find out easily with the Show Project Copies command.
To unlock a master document from the master document itself:
The red lock icon next to the master document in the Navigation view will disappear and its Locked property will be set to No indicating that the document has been unlocked.
Related concepts
Understanding project definitions
Related tasks
Copying a master document to a project
Showing master and project copies
Unlinking from a master document
Releasing a project copy as a new master document