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.
To view the versions of a sheet:
Click on the File menu
Choose Version History
Choose See Version history
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 revert the sheet to the revision currently showing, click on the Restore this version button along the top of the screen
To return to the sheet, click on the Back arrow near the top-left corner of the screen.
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:
Clic on the File menu
Choose Version History
Choose Name Current Version
Type a name for the version and click on Save
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.