Live Spreadsheets keeps a complete history of all changes to your spreadsheet and you can easily revert back to any version - either at the spreadsheet level or at an individual cell level.

Right-click on a cell and in the menu options, you will see an entry saying "Revision history". Just click on the revisions button to bring up a panel where you can see all the iterations of the cell with associated information like who created that cell version and when - from here, you can select any historical version and cilck on the "Revert" button to go back to that version.

You can also see all the revisions on a spreadsheet by clicking on the "Revision History" menu option in the top menu (under the tools tab).

If you would like to hide all revisions to your cells for privacy or confidentiality reasons, you can also choose to disable other users (apart from the author of the document) from viewing the revision history.

In addition to cell-level history, you can set "save points" in your document where the entire contents of the document at that particular point in time are archived - you can restore the document to any of these save points at any time.