This means to do something again. You change the thing you are doing it to. Or you change how you do it.
Verb [stem] + 替えるPrimary
Verb [stem] + 換える
This means to do something again. You change the thing you are doing it to. Or you change how you do it.
この駅で次の電車に乗り換えてください。
Please transfer to the next train at this station.
家に帰ってすぐに部屋着に着替えた。
I changed into my loungewear as soon as I got home.
このレポートは内容が古いので、全面的に書き替えないといけません。
The content of this report is outdated, so it must be completely rewritten.
先ほど言い換えた表現の方が、より的確だと存じます。
I believe the expression I just rephrased is more appropriate.
新しいソファを買ったので、古いものと入れ替えるつもりです。
I bought a new sofa, so I plan to replace the old one with it.
Imagine you have two boxes. One box is for things that are the same, but new. This is like '替'. The other box is for things that are different. This is like '換'. Sometimes, the boxes can be used for similar things.
This is different from 〜直す (naosu). 〜直す means to fix something to make it better. 〜替える means to switch something for a new one. For example, 書き替える (kakikaeru) means to rewrite with new content. 書き直す (kakinaosu) means to correct what you already wrote.
Don't use this when you want to change just anything. For example, you can't say "飲み替える" for "change drinks". You use it for specific verbs. These verbs show you are swapping one thing for another. Think of changing clothes or changing trains. It is for replacing something.
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