Every time a file is added, edited, deleted or restored, a new version of the file along with size, timestamp and the user taking the action is recorded. You can open and view any version for a file and revert to any of the older versions.
Navigate to any file that you wish to view its versions. Tap the context menu for that file then tap Versions.
Tap the version you wish to view or restore.
Tap Open this Version to view the previous version of the file. If you wish to restore this version, tap Revert to This Version. This version of the file becomes the latest version, replacing the current version. If you wish to save the current version, open it, make a minor edit then save it. This creates a new version which is the one that will be replaced when you restore the older version.
The Revert to This Version option is not available for the latest version of the file or if you are not the owner or have the Editor permission.