Fix is often used to talk about repairing a vehicle or a machine. Mend is often used to talk about repairing holes in clothes, shoes, roofs or roads. It can also be used to talk about repairing anything that is broken.
Our car isn't working, but Dad will fix it soon.
Oh, no! The computer got stuck again. Can you fix it?
The little boy fixed the brakes on the bicycle by himself.