From your self-serve select plans and add-ons then select change number
Then select if you want a new number or if you'd like to port in
Note you can change your number once every 30 days
Even though the answer have been give, I am going to give a plug to the help pages. This is what I get when I search for change phone number. The article links are very informative. I know I am putting myself out of a job by suggesting the help search. It does work very well.