Normally you restore home assistant to a previous backup using the backups stored locally that are listed under Settings -> System -> Backups. Just click on the backup you want to restore and a window pops up. Simple.
But if you want to restore to a version saved to Google drive you first have to copy the backup file from Google Drive to your local PC and then upload it to Home Assistant. Then the feature to upload a backup file is hidden under the “three dot” menu at the upper right of the backup page.
Open the menu, select “Upload backup”, and find the backup file you copied from Google Drive.