Whenever you save your watch face, Facer will keep track of that version so that you can revert to it whenever you need to.
If you make a mistake or need to dig up an old idea, you can select any previous version and set it as the current version. Did you not mean to pick "that" version? It's ok, nothing is ever lost! You can still revert back to the version you started with.
Just look for the icon in the top bar that will let you access the dropdown with your versions.
Here is an example: