Use a mixture of half water and half white vinegar. the vinegar will "unset" the crease. I have used this mixture for ages...started at age 14...to take the crease out of pantlegs for my brothers. Also when I took in pants the perma press crease would be in the wrong place...so I just spritzed with the mixture, ironed out the old crease...spritzed and ironed in the new one.
I use it on fabrics every day. Check for color fastness on reds and blacks...but you do that with water anyhows.