Remove (unscrew) the format scale on the right rear of the upper bellows. Keep the screws and scale - you may want to put it back
Unplug the enlarger.
Remove the existing head by pulling forward on the two sliding clamps either side of the head. It helps to do this with the head at chest level. The existing head should lift off. If it is a condenser head, I suggest keeping it level - I have known the condensers to slip.
Drop the 45 head in place, and slide the clamps back to lock it down. They will be tight, but should move. You can adjust the tension on the clamps.
Depending on the model of head and chassis, work out which plug goes where.
Both heads have glass or diffusers close to the bottom rim. Put them on clean flat surfaces to avoid damage.
Graham