I have an old acoustic guitar and one of the tuning keys is bent so I can't tune it at all. Is it very costly or difficult to replace tuning keys?
i don´t think it would cost so much and it isn´t hard, but go to a store...you may screw up the guitar otherwise
Ya, go to a store and get it replaced, it's really not that expensive.
Why go to a store? Can you get a picture of the back of the headstock? Is it steel or nylon string?