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.
When you open a previous revision and make a change, you will be presented with two different options when you attempt to save (or if autosave triggers).
Clicking continue will replace your "Current Revision", meaning the initial revision you had opened before switching to this one will be replaced. You can avoid this by making a separate copy of the revision you most recently switched to.
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.