This article explains how to change the watch face on a Wear OS device such as a Google Pixel. The instructions in this article apply to all Wear smartwatches and wearables.
Sync your first WearOS watch face to Facer
This is the easiest way to change the background of your smartwatch:
Touch the screen to wake up your watch if the screen is dark.
Tap and hold anywhere on the screen background until you see a list of watch faces to choose from.