Users with administrator access can change a member's mobile number themselves in the dashboard.
Open the dashboard and click "Members" in the menu on the left
Search for the member
Click "Edit" on the far right of the row
Change the mobile number in the customer profile
Click "Save profile"
Don't see the "Edit" button? Then your user doesn't have administrator access. Ask an administrator in your company to make the change, or to give you the right access. Read more about roles and access in Step 8: Roles, Permissions & Team Setup.