Refers to the act or process of changing something; often used in compounds or as a nominalized form of 変える.
I went for a walk for a change of pace.
He won't change his mind.
替え (same reading) usually means a spare or replacement item (e.g. 替えの靴下), while 変え refers to the abstract act of changing something.
change; alter; transform · reform; revise; amend
Nominalized form of the verb 変える (かえる, 'to change'). The kanji 変 means 'change' or 'strange'.