To change the watch mobile number in the App;

Go to the Main App screen and Tap Manage

Tap the watch mobile number at the top of the page

Tap on the watch mobile number again on the newly opened page

A box will appear with the mobile number visible allowing you to change the number.

Add the international code of the mobile number of the watch sim card in the first box such as +34

Add the rest of the number into the second box

Once you have completed the change, tap confirm