Customers phone numbers may change over time and need to be updated in the system. You can do this at Take Order while putting an order through, but sometimes you aren't putting through an order and still need to update the phone number.
Navigate to Orders > Customers > and search for the customer you wish to update and click on their customer profile.
Once in their profile find Addresses on the left side menu. Next be sure you are selected on the "Default Billing Address" and then find the Telephone field on the right near the bottom.
Change the number to the new information, and be sure to click Save Customer!