Once your HTC One or HTC One mini is updated to Sense 5.5 (which comes out of the box on the HTC One max), you can access your free storage. To redeem the offer, open the Google Drive app. Press "Redeem" and then follow the screen prompts. Google suggests that you have a Wi-Fi connection when redeeming the storage offer, which is good until January 1st, 2016. Once your two-years is up, does your data turn into a giant pumpkin? Nope, it remains available in Google Drive for you to access, download, or share.
There are some rules.You can redeem the free offer one time for each device. Using the same Google account you can redeem one +50GB offer and one +25GB offer on two eligible devices. If you need more storage, it can be purchased from Google. Also keep in mind that Google offers 15GB of free storage with Google Drive right off the bat, which includes your Gmail account.
source: Google via AndroidPolice