I bought cs6 for pc years ago . . now have mac and would like change over and upgrade (perhaps bridge only) please advise best?
No way. You are long past any upgrade eleigibility cycles, so you would have to buy a full CS6 license or join Creative Cloud. That also negates any advise people could give you on making a platform swap, which requires you to be on a current version, anyway.
You can either buy CS6 or a subscription. A cloud version allows you to use either OS.