How do I change the face of my watch?

If you have multiple faces already defined, you can just swipe firmly from left right or vice versa. It is easiest to do by starting the swipe from the watch edge already. That will let you flip thru each of them.

And to set up multiple faces - if you want that - open the Watch app on your iPhone and tap on Face Gallery. More details here:

Using the Watch app on iPhone:

Explore the Face Gallery on Apple Watch - Apple Support

Or doing it from the watch directly:

Change the watch face on your Apple Watch - Apple Support