To Be Renewed
To Be Formal, To Be Altered, To Be Revised
intransitive verb, godan verb
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 renew so the verb vocab version is to be renewed. Like most verbs ending in まる, this is an intransitive verb, meaning it happens all on its own. Nobody is doing the renewing, it's just happening (as renewing often does).
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You probably learned this reading with this verb's transitive other half 改める, but here's a mnemonic anyway just in case:
To be renewed you decide to go to a spa. You get in a bathtub and they pour in a lotta (あらた) leeches and eels to help you to be renewed. Feel the fear. There are a lot of them! Alternatively, you can imagine them pouring in a lotta Maru cats, because that's way fuzzier.
まさか日本の年号がワニカニに改まる日が来るなんて！
I never thought that the day would come when the Japanese year would be altered to WaniKani.
そう改まるなよ。うちの社長は、とても気さくなんだから。
Don’t be so formal! The president of our company is very friendly.
その山の上で空を見上げると、心が改まるような気持ちになりました。
When I looked up at the sky from the top of that mountain, I felt my heart become solemn.
アイツの悪いクセがすぐに改まるとは思えないよ。
I don't see his bad habits changing anytime soon.