Job at next

So basically I'm 16 and due to start a temporary contract job at NEXT tomorrow.

I just received a phone call about my first shift. They didn't even bother to tell me they had changed my shift pattern/times already.. but then she asked if I could work even more hours than they already had me down for.

I've just done my gcses and I'm stressed out to the Max and I honestly just said I'll do the hours asked and got off the phone and just cried my heart out. I've had a lot on my plate recently.

How do I go about quitting next? 😣 without talkin to them face to face or over the phone if that can be avoided? I'm not the most confident when it comes to this sort of thing...