"To see revision history for a file, follow these steps:
Open a document, spreadsheet, presentation, or drawing.
Click the File menu and select See revision history.
Click a time stamp in the panel on the right to see a previous version of the file. You’ll also see the people who edited the file below the time stamp, and the edits that each person made is shown in the color that appears next to their name.
If you want to revert to the version you're currently viewing, click Restore this revision.
If you'd like to return to the current version of your document to continue editing, click the X in the upper right of the 'Revision history' panel."