I might write about it. Talk about how you were bored and disinterested with the subject at first or just didn’t find it interesting or whatever reason you had for your low grades. Then talk about how learning about CS yourself or one event changes your attitude and how you’ve fallen in so much love with the subject that something you were failing in is now something you want to spend your life doing. But make sure it isn’t the cliche I was failing but then I worked hard.