Every revision of a file creates a new file which is never deleted or modified once created.
To view the history link, you must be logged in.
In the history you see each version, the timestamp, the status of the revision and the user that created the revision.
If you click on a revison, you see a diff in wikitext. Inserted text is green, deleted text is red.
You can then revert to that revision. This will create a new revision with the text of the reverted revision.
NB: Note that the History is a query. You might have to request the page multiple times to get all revisions.