Is the 'bow' in the neck area alone or is it in the join between the neck and the body? If it's in the neck alone there is possibly a simple solution. If it's in the join between the neck and the body then the guitar needs what is called a 'neck reset' and that is a big job, usually only worth doing on a top class guitar.

A bow in the neck alone can be fixed fairly easily if the fretboard was originally glued on with hot hide glue. It needs clamps, a strong straight edge and heat lamps. Both of these jobs are best done by a luthier and probably won't come cheap.