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: Your status is to be decreased amongst the eyes of the Gods. They decide your punishment is to be decreased into hell (へる) where you will live the rest of your days.
My weight decreased by two kilos in ten days.
WaniKani sales decreased by 5% this month.
My right shoe always gets worn down at the heels faster than my left one.