How do you fix headphones?

It does depend on what the problem is. The most common thing to go wron on headphones is a break in the cable near the jack. To fix this you can cut the cable and solder on a new jack.

If it is not a break in the cable near the cable it is sometimes a cable has come loose inside the earcup... This can be tricky to fix but be brave open them up and you may see the problem.