By default, WeVideo keeps snapshots (revisions) of all changes made to a timeline every time you save.
To access your previous revisions, click on the Menu button on the top left of the editor and then select the Revisions option.
Here you will see a list of all the previous revisions tied to this timeline.
If you are working in a collaborative project you will also see revisions by other authors in your project.
Click on a revision to see a preview and then click on Open to load that revision.
Please note that if you are reverting to a revision that used media which is no longer available to your account, the revision will load without the media.