Colloquial abbreviation of 'image change' — changing one's look or impression (hairstyle, clothes, makeup), i.e., a makeover.
春だし、ちょっとイメチェンしてみようかな。
It's spring — maybe I'll try changing my look a bit.
友達が髪を染めてイメチェンしていた。
My friend dyed her hair and changed her look.
このアプリでメイクのイメチェンを試せるよ。
You can try a makeup makeover with this app.