改心 (kaishin: to rectify + heart, mind) = conversion, reform
彼は改心する決心した。
Kare wa kaishin suru kesshin shita.
He resolved to turn over a new leaf.Did you notice all the hearts (心) in this sentence?!
彼 (kare: he)
決心 (kesshin: determination, resolution)
to decide + heart, mind