You can change your phone number from your profile section.
Please follow the instructions here: https://docs.cashfree.com/docs/manage-account#update-phone-number
Please note that you will be asked to verify your identity through Aadhaar OTP validation, so make sure that you can access the Aadhaar and Phone number that was used during the activation process. Please write to care@cashfree.com if you require further assistance.