Should I kasher my retainer for Pesach if so how should I do it? I can’t put hot or warm water on it because the retainer is plastic and will melt
They should be cleaned well with whatever soap or cleaning solution you use. You should not eat hot chametz in them for 24 hours before Pesach. Since any kashering will ruin them you can rely on this procedure.
See Shevet Halevi 1:148, 4:74, Yesodei Yeshurun Hilchos Pesach pg. 141
also see here