From now until Sunday, you can trade in your old smartphone for as much as $300 (with a minimum of $100) toward the purchase of the HTC One from T-Mobile. To start, you need to find out how much HTC will pay you for your phone. That is as simple as clicking on the sourcelink below and entering the pertinent information. Once you get a quote back from HTC, there will be a personal ID number that you will need to save.
For example, taking our old BlackBerry Storm, we punched in the information and not surprisingly, we were offered $100 credit toward the purchase of an HTC One. Trade in a 16GB Apple iPhone 4S and receive $270 toward your HTC One. The HTC One itself must be purchased between June 5th and June 9th, but one you've filled out the online form, you must mail in your trade along with the proof of purchase for the HTC One by June 30th.
source: HTC via AndroidAuthority