This is the 2159th most frequent Japanese word.
替え
"替え" means "substitute" or "replacement" in English.
Here, '替え' is used as a noun meaning the substitute or replacement.
ズボンを新しい替えにしました。
I changed my pants to a new pair.
In this case, '替え' is part of the verb '替える', meaning 'to replace' or 'to exchange'.
古いものを替えて新しいものを買った。
I replaced the old one and bought a new one.
Here, '替え' refers to a spare or replacement item, in this case, clothes.