You are unable to directly update the number associated with your Sitemate account. Instead, you'll need to logout of your current account & re-sign in using your preferred number. This method essentially creates a new login, using the new mobile number.
Steps:
Go to the Sitemate App & click on the Profile tab
Scroll down and select Logout
Once logged out, click Start & enter your new number
Follow the remaining prompts to finalise your account set up
Notes:
If you had ongoing access to certain Sitemate Links, you'll need to re-enter the link to access the forms again.