This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means substitute, and the verb 代わる means to substitute for or to take over for. Use it when one player substitutes for another in sports, for example, or when someone takes over for you at work.

代わる is usually intransitive, but sometimes it can be transitive and take a direct object. Kind of like "to take over" in English — someone can take over your shift (transitive), or just take over for you (intransitive).

Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you to remember the か part:

You need something to substitute for your car (か). It's getting repaired, so you can't use it now. I guess you can substitute your bike for your car?