I'm not sure if you mean you don't know how to change it, or you have been trying but are getting an error.
Here is how to change the clock face:
How do I change my clock face?
To change your clock face:
- With your device nearby, in the Fitbit app, tap the Today tab
> your profile picture > your device image. - Tap Clock Faces > All Clocks.
- Browse the available clock faces. Tap the clock face you want and tap Select to install it.
- Sync your device to see the change.
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.