Loosen the Phillips screw on both sides and remove the twist tie on the cables. You may need to loosen the wafer head screws at the top end, just above the Phillips screws, to get it to tilt. If it does tilt, look from below to see if it has any kind of release that needs to be slid down or rotated- I don't think it will- the machine screws on the top rail will prevent lifting it off, so you'll need to use a long Allen wrench to back them and it probably has a bend in the bottom rail for the bottom screw you mentioned.

Show the other side of the mount, if possible- I have seen some that have a piece of steel that slides across the top rail, to prevent it raising, but they usually don't have the long screws at the top.