Updating your phone number or adding an alternative phone number can be done by heading to the business profile page on your business dashboard.
Click on Maintain Profile, then click on Update profile. This shows you your business details page, click on edit beside page lets you update your phone number.
You can also add add an alternative phone number by editing your contact details.
Note that till you verify your business with the OTP sent to you, your business profile will not be updated with the phone number.