On PetCloud, for security reasons, your mobile number can only be updated by a PetCloud Support Team member, once they verify you, after you have created your account.

Why? Your registered mobile number is what authorises withdrawals from your PetCloud wallet. So if an unauthorised person unlawfully gained access to your account, they would not be able to withdraw payments into another paypal account because it requires your authorisation, with a 6 digit code smsed to your mobile number, which you then put into the screen to authorise.  This method is known as '2FA' or 'Two Factor Authentication'.

The location of your Mobile number is in the Profile screen.