Lets take a logical approach to this question, if we may. This is a "which came first, the chicken or the egg?" How do you suppose to change the mobile number WITHIN THE ACCOUNT, if you cannot gain access to the account to change it?
The answer surprisingly seems to be, YOU DO NOT! It is LOST. Make sense? we are CCm.NET, a technical forum for technical problems. This is an account management issue, and we cannot assist with this type of problem, beyond posting links. In this case, no link will assist.
We understand that this is too late in this circumstance, but always create IMPORTANT accounts, such as email, and lesser important accounts, such as social media, on a landline workstation. A landline workstation can be laptop on wifi, or a desktop on wifi for that matter. The important thing; the account is created from a WIRED, NON-cellular network.
The provider can offer many ways to recover your account, if you create it in this manner. Always attach a recover email, such as a work, or another email that you will not forget the username and password to (because it SO important)!
After you have attached another email address in case of forgetfulness, now you can attach a mobile device to the account. Now, if you lose, break, or stop having access to your mobile device (non-payment of services), you can still recover your account!