If you've made a change to your template that you want to undo, there's an option for that.
How to undo a change to a template
In the upper left corner of the screen, underneath the Save button, you'll find the undo button.
It looks like an arrow pointing backwards. Click the undo button when you want to undo your last design change. You can undo multiple changes with repeated clicks.
Note: you cannot undo changes after you've exited the design area. Also, there is no redo function so make sure you actually want to undo the change before doing so.