Major numbers are meant to keep track of official project releases. For this reason, you cannot change the current major version number for individual files, but only for an entire project at once. You can do this by selecting the New Major item in the Action menu after you have selected a project. This action can only be performed if there are no locked files in the project.
The New Major action creates a new identical revision of each [current] revision in the project, makes the copy current, and then assigns it a version number with a higher major, and a zero minor. For instance a current revision may have the number 1.5. A new major action will create a copy of this revision with the number 2.0.
Note that the new major applies to all new revisions. If you make an earlier revision with an old major current, and edit and unlock it, the new [current] revision will have the current major, not the old one.