There's 3 ways to do it. Using Garmin Express, Garmin Connect Mobile on your phone, or using the new Connect IQ app for your phone.
Here's a basic "how to" for Garmin Express and Garmin Connect Mobile, but it appears the images didn't make it through when things were moved to the new forum, but the text should help.
I think the description in that forum post for Garmin Connect Mobile (GCM) on Android is out of date. For watch face settings in GCM Android these are the menu options I navigate through:
Go to your device (either by clicking on it directly at the top, or via "Garmin Devices"
Select "Appearance"
Select "Watch Faces"
Select the specific watch face
Select "Settings"
Garmin Connect IQ Store app (GCIQS) on Android is similar:
Go to your device (either by clicking on it directly at the top, or via "Manage Device"
"Appearance" shows for devices with music but not for non music devices, and other things are a bit different with the Android vs iOS version of GCM. It's something I reported about a year back. I've also suggested that that thread should been re-written as a "wiki" here There must be hundred of apps in the store that reference it in the old forums, so no longer accessible, so a good time to change..
