You can go to the Mobile tab of your Account Settings page to add, remove, or switch numbers.
If you already have a mobile number on the account, you can click "Add another phone" to activate another number for Mobile texts, or remove your existing number by clicking "remove" next to the number you wish to remove.
If you have more than one number registered on your account for Mobile Texts, you will have the ability to choose which number you want to send your Mobile Texts to. Simply select the mobile number you want to receive your texts to in the "Send texts to" drop-down menu at the bottom of this page.