From the home screen of the Frontpoint App:
Tap the Menu button
Select the My Account section.
Tap on the pencil icon next to Account Info. Here you can edit both the Primary and Secondary phone numbers.
Tap Save when you are done.
Note: This section of the Frontpoint App will only appear if you are logged in using the primary login for your account. Any secondary logins you have created will not have access to this page.