I want to unpledged my allegiance and choose another that my friend is allied to can I and if so how. must I be in the system I first pledged and if so how to work out where to go?
There should be an option to "Leave Power" on one of the power screens. You have to wait a day, but then you can pledge to the power your friend is pledged to.
In the actual PowerPlay screen. You can access it from the Main Map or the GalNet button in the left-hand cockpit panel.
You can also switch sides immediately if you're willing to have your former power hunt you down for a few days - select the Power you want to switch to and go for Defect.
Oh and maybe this should be in the PowerPlay section?