how do I switch voltage on amp?

Are you certain it has a dual voltage transformer? If it does; whether you can, would depend on your skills at rewiring a transformer's primary(generally, from series to parallel). Easy, with experience and a color code, or numbered leads.

The common colors on a dual primary transformer are Black White Brown and Orange. For 220V operation Phase is applied to the black lead, white and Brown are connected together and Neutral is connected to the Orange lead. For 110V operation Phase is applied to both Black and Brown and Neutral is connected to both White and orange.

If you have no experience with this I strongly encourage you to have someone with experience do this for you, even if you have to pay for it you will be WAY ahead of the game with the steal you made on that NRG :-)

Good Listening

Peter