In your self serve account on the right side there are a few links, the last one is "Change Number", you can also go into the "Plan & Add-ons" Tab and see the option there.
Hey, It's actually still available if you look here: http://productioncommunity.publicmobile.ca/t5/Announcements/UPDATE-Only-4-days-left-to-get-our-limited-time-4GB-40-promo/m-p/77350 "Note: The promo plan has been removed from our website; however i...