how do you reinstall a watch face deleted by mistake

You can't actually delete any of the installed watch faces. You could only delete one that you've customized. There's no way to undelete that. You'll need to follow the directions above to re-customize.

1. Force Touchon the clock face to go to switching mode.
2. Next up, you will need to swipe all the way to the right and then tap on + New
3. Now, simply rotate the Digital Crownin order to select the clock face which you want to add.
4. Up next, just tap on the clock faceto set it as the new watch face.
5. Now, you can customize the new watch face as per your liking.