iFixit

Simultaneously push the two plastic clips on the far left and right sides of the I/O board toward the middle of the I/O board and pull the I/O board slightly away from the outer case.

Pull the I/O board/logic board assembly out of the outer case a maximum of 5 mm. Pulling the assembly out further may damage the IR sensor connector (highlighted in red in the third picture).

Lift the hard drive from the edge nearest the logic board and remove it from the mini, minding the RAM socket and any cables that may get caught.

When reinstalling the hard drive, be sure the two T8 Torx lugs on its far end are inserted into the two rubber grommets highlighted in red in the second picture before sliding the logic board back into place.