Log into Delta and go to your profile and edit it there. Historically the DL system did not maintain a separate field for middle name (it does now), which is why they would concatenate the first letter of of middle name to the end of the first name and put that into the first name field. The PNR (Passenger name record) would store "Paul R" as my first name. With the advent of TSA and passport information it was necessary to capture the middle name, which is required for your passport.

As reported here, and experienced personally, if the PNR name information does not exactly match your passport or trusted travel name, you will be denied TSA PreChek. Which is why you should always check your name and information at least 4 or more days prior to travel to make sure it matches. Changes done in less than three days will disallow TSA PreChek.