When you want to modify a file maintained with version control, type
C-x C-q (vc-toggle-read-only). This checks out the
file, and tells RCS or SCCS to lock the file. This means making the
file writable for you (but not for anyone else).
If you specify a prefix argument (C-u C-x C-q) for checkout, Emacs asks you for a version number, and checks out that version unlocked. This lets you move to old versions, or existing branches of the file (see section Multiple Branches of a File). You can then start editing the selected version by typing C-x C-q again. (If you edit an old version of a file this way, checking it in again creates a new branch.)