Contact one of their reps at the phone number on your bill or 1-800-Comcast, chat with them using one of the options at https://support.xfinity.com/chat, or visit a Service Center if one is nearby (check locations and hours at http://www.xfinity.com/support/service-center-locations/). Have them add HD service to your account. The cost is $10/month. If any of the cable boxes where you want to view HD channels are not HD boxes, you'll also have to swap them out, no extra charge.
Comcast sometimes responds to questions here in the forums, and sometimes not. When they do it might be in a few hours, or it may take a few days.