i see the o2 sensor at the back of the engine, the power is easy to disconnect , but what is the easiest way to take the sensor itsself out to replace?

Use an oxygen sensor tool and a ratchet. The links are pictures of two different types.
http://www.autopart.com/tools/toolsmain/tool/T_2065.htm
http://www.autopart.com/tools/toolsmain/tool/T_2066.htm