Revision history is available on Premier accounts. File revisions are stored on each save. You can access a files revision history from
File -> Revision History
On the revision history window, click through the revisions on the left hand side.
You will see a diff of the revision compared to the current file.
You can then click "Restore to current" to overwrite the current file.
You can delete unwanted revisions by pressing the delete key.
Even if a file has been deleted you can still access it's revisions.
From the revision history window click the file dropdown and select the neccessary file.
Then click "Restore to new" open as a new file.
By default shiftedit will save up to 20 revisions per file. This can be modified in:
Site -> Edit Site -> Advanced -> Revisions per file