To switch between branches, type C-u C-x v v and specify the revision ID you want to select. On a locking-based system, this version is then visited unlocked (write-protected), so you can examine it before locking it. Switching branches in this way is allowed only when the file is not locked.
On a VCS with dot-pair IDs, you can omit the minor part, thus giving only the branch ID; this takes you to the head version on the chosen 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.