To switch between branches, type C-u C-x C-q and specify the version number you want to select. This version is then checked out unlocked (write-protected), so you can examine it before really checking it out. Switching branches in this way is allowed only when the file is not locked.
You may omit the minor version number, thus giving only the branch number; this takes you to the highest version on the indicated branch. If you only type RET, Emacs goes to the highest version on the trunk.
After you have switched to any branch (including the main branch), you stay on it for subsequent VC commands, until you explicitly select some other branch.