Creschy

Let's say I disenchanted something with 20% firedmg-reduction but I found something that has 50% dmg reduction. The problem now is that I can't disenchant the new one because I have already disenchanted something with firedmg-reduction and I would like to know if any of you know a way to reset my old disenchants or overwrite them with the new one? I would appreciate it if someone could help me out with my problem.