2002 Buick Century camshaft poition sensor

I get terrible gas mileage, intermittent start, and recently rough idle. The engine light is not on, although it came on once so I hooked up my scanner and had the code p0340. I looked it up and it is the cam sensor. I have the Haynes manual but it doesn't say how to test/replace the cam sensor. Any help on the matter would be appreciated. Maybe a diagram of how to replace it? Thank you.