First, make sure that you’re using the same Google account on your watch and phone. Next, go to Google Play on your smartphone and search for “watch face name”. Tap the down arrow next to “Available on more devices” and select “install” on “your watch”. Once the watch face is downloaded, long-tap on the current watch face, swipe left, and select “Add watch face”. Keep swiping left until you see your newly downloaded watch face, and then tap it to make it your active watch face.