When editing a template version, you can easily undo and redo changes. The undo and redo change buttons are located on the right side of the formatting toolbar.
To undo the prior change. Simply click the Undo button. This will step back the last change. You can keep on clicking the undo button to revert multiple changes. But you can only undo changes since the template editor was loaded to edit the version.
You can see in the above screen capture that "Add More text here" is in the template. Once the Undo button is clicked that text is removed since that was the last change.