Every change made in the Timeline view is tracked as a new version of your channel in the Change History dropdown list at the top of the page, to the right.
Selecting the History icon will display the entire change history for your draft channel, with the most recent changes appearing at the top followed by older changes in descending chronological order.
The list will also show a brief description of the change, the author of the change, and the timestamp of the change. You will see an icon to revert to a previous version here as well, see Reverting to a Previous Version.
A channel’s Change History is dependent on what state your channel is in. Each time you enter “Edit Timeline” your channel is in a draft state until published. Once published, the Change History versions are reset. You will not be able to revert to any “pre-published” draft versions of your channel after it is published. You can, however, edit a published channel at any time by selecting “Edit Timeline” from the All Channels view. This will create a new draft for you to work from, and begin a new list of versions in your Change History.
Your new draft must be published in order for changes to be reflected in the published state of the channel.