I need to play a song of which I only have the piano score, in a guitar. Is there a method to transpose piano?

Answer 1

Both guitars and piano play in C, there's no need to transpose if you're playing with both guitar and piano.

If you need to transpose you will need to rewrite the music or transpose on the spot (not so hard if you're going up or down a tone, otherwise it's usually pretty tough). If you're playing with a Bb instrument from the same piece of music you will need to play down a tone - if the piece was in C you would need to play/write/transpose it into Bb, or the other instrument would need to transpose UP a tone into D.

There is no practical way to transpose an acoustic piano. You would need to get your piano retuned, expensive and bad for your instrument - piano strings are under HUGE amouts of tension - several tons. Transposing up a tone may well cause your piano to implode as the strings pull it apart. You could play it on an electric piano - almost all of these have a transpose button to transpose it easily.