Change dimmer switch to regular
Pigtail time

You'll need a couple lengths of the same gauge wire (preferably black, but any color other than white or green will do here) as what's in the box right now to serve as pigtails; if you don't have some NM laying around to shuck for this, just get a foot or so of 12AWG black THHN.

Once you have the old dimmer removed, one of them gets shucked of its insulation and goes between the existing ground bundle in the box and the green screw on the new switch, the second pigtail goes from the existing wirenut with the red and black wires in it to one brass screw as that's your always-hot, and the final pigtail goes from the existing wirenut with the single black wire in it to the other brass screw on the switch as that's your switched-hot (NOT "neutral" at all -- the neutral is the white bundle in back that you aren't touching).

Button it all back up, turn the breaker back on, and enjoy your new fan!