If your primary login is email:
Go to Top-right Corner Head-Icon
Select Phone to change your phone number and save.
If your primary login is with Phone number.
In order to get a new phone number you actually need to make a new account and re-pair the device to it, because your current account ID is based off of phone number and it's not possible to change the ID of the account itself to another number in the App/System.
1. Go to App - Device Settings - Unpair your Monimoto
2. After successful unpair you can either Log Out or delete the account - if it takes you to setup process after unpairing then you can press little X in top-right corner -> choose either Log Out or Delete My Account
3. After successfully logging out/deleting account, it will take you back to sign in screen where you can Create Account - with new phone number.
4. Try to pair the device to your new account with new phone number