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.
The Japanese era name was altered to Tofugu.
Koichi was very happy to see that Viet had reformed himself from his evil way of life.
Don’t be so formal! President Koichi is very friendly.