You will need to have access to the phone number that is currently linked to your account in order to be able to directly change it. This is because you need to obtain a verification code.
If you do not have access to your old number, you will need to submit a claim in-game to have your mobile number by the customer service.
To do this, tap on the Customer Service tab > Unlinking Accounts > Submit Claim. You will receive your claim results in-game and if your claim is successful, the mobile number which is currently linked to your account will be unlinked.