1To take over someone's role, duty, or shift from them - replacing or relieving them.
夜勤の担当が同僚と替わった。
The night-shift duty was handed over to a colleague.
部長が異動になり、後任として私が替わることになった。
The department head was transferred, and I was chosen to succeed him.
2To act in someone's stead or on their behalf; also used for handing the phone over to someone else.
社長に代わってご挨拶申し上げます。
I will give the greeting on behalf of the president.
母が出かけているので、私が代わって電話に出た。
My mother was out, so I answered the phone in her place.
少々お待ちください、担当者に代わります。
One moment please, I'll hand you over to the person in charge.
3To swap places or roles with someone; emphasizes mutual exchange.
電車で席を換わってもらえますか。
Could you switch seats with me on the train?
二人はじゃんけんで役割を換わることにした。
The two of them decided to switch roles based on rock-paper-scissors.