When you are working in Google Sheets, all changes will be recorded, regardless of who they were made by. This allows you to view old versions and revert to an earlier version if changes have since been made in error. 

View sheet revisions

To view the versions of a sheet:

A list of all major revisions will display on the right-hand side of the screen - click on a revision to show the sheet as it was revised on that date:

To return to the sheet, click on the Back arrow near the top-left corner of the screen.

Naming a sheet version

You can create named versions of your sheets, to make it easier to identify a specific version of the file. 

To name the current version of the file:

You can click on the ellipses next to a version of the sheet to name that version or make a copy of that version of the file. This allows you to keep an old version of the file without losing the changes made to it after that version.