To change the number in Yolla, you need to sign out of your current account and register a new account with a different number.
iOS:
go to Account
Tap on the account name
choose Log Out
Android:
open the side menu
tap on the account info area
choose Log out
If you'd like to transfer the credits from your previous account to a new one, please send us any payment receipt you have for any previous Yolla payment to verify your account (they usually arrive by email after you make a payment).