if my iphone is broken, how do i back it up from my macbook air?

If my iphone is broken, how do i back it up from my macbook air?

My screen is broken and water got in it, the screen is black and won't turn on but it still rings when I call it and stuff. I want to put my sim card in another phone I have but my phone hasn't backed up to the iCloud in a few months. Automatic backup is on, I know that and I can see when it last backed up on my computer but I want to know if there is an easier way to make it backup.