First of all, create a new account using your new number.
If you are the only guardian of the watch and you still have access to your account, add your new number as a second guardian. Click here to learn how to add a second guardian. You can then delete the old account and become the only guardian of the watch.
If the watch has two guardians, ask the other guardian to delete your old number from the contact list and then to reassign you your guardian's rights.
If you'd rather wish to start everything from the beginning, we can factory reset your watch and delete your old user account, so that you can start the activation process from scratch.