How do I fix this?

Many modern day cars don't have much extra length on the cables. As others have said, if there is, cut the insulation back to get in a replacement clamp battery end. But most likely you're going to end up buying new wires along with the battery. If you're not familiar with what you're doing you should take it to a trusted mechanic. Don't want to reverse or short out any wires. Batteries carry a lot of energy and you could harm yourself or damage electrical equipment in the car if you're not familiar with what you're doing.