1To replace something with another (newer) one — a worn part, a battery, wording, etc. Usually written 替える. Distinct from 変える, which means to alter the nature or state of something itself.
リモコンの電池を新しいものに替えた。
I replaced the remote's batteries with new ones.
汗をかいたのでシャツを替えた。
I'd worked up a sweat, so I changed my shirt.
同じ言葉が続くので、表現を替えて書き直した。
The same word kept repeating, so I rewrote it with different wording.
2To exchange or interchange one thing for another — exchanging currency, letting in fresh air, swapping seats, etc. Usually written 換える.
空港でドルを円に換えた。
I exchanged dollars for yen at the airport.
部屋の空気を換えようと窓を開けた。
I opened the window to change the air in the room.
ためたポイントを商品券に換えた。
I exchanged the points I'd saved up for a gift voucher.
3To substitute or replace a person or role with someone else — substituting a player, changing who is in charge, etc. Usually written 代える.
監督は後半、けがをした選手を代えた。
The manager substituted the injured player in the second half.
この役は彼に代えられる人がいない。
There's no one who can replace him in this role.
言葉に代えて、深く頭を下げた。
In place of words, I bowed deeply.