Scroll down and tap Unpair device or Factory reset (wording may vary by app version).
Select your Xiaomi Watch S1 from the list. How to Hard Reset XIAOMI Watch S1
Remember to sync your data beforehand, and within minutes, your watch will be ready for a fresh start. Scroll down and tap Unpair device or Factory
Tap the green checkmark (✓) to confirm, or the red X to cancel. and within minutes