How to replace upstream O2 sensor

I need to replace upstream O2 sensor on my Subaru Forester L 1999. Do I need to remove the shield under the catalitic converter or it's possible to unscrew the sersor with some kind of "socket" wrench ? All the bolts which hold the shield are very rusty and I'm not sure if it's possible to unscrew them.

The upstream H02 sensor is the sensor closer to the exhaust manifold, it's the size of a spark plug, and canbe taken out with a wrench.

Danielsatur (http://www.automotiveforums.com/vbulletin/member.php?u=559976),
Thank you for the answer. I just want to make sure that it should be some special/socket wrench. Shouldn't it ? It's impossible to unscrew upstream O2 sensor with any regular wrench.

Disconnect H02 sensor, thread a good boxed end wrench, and turn counter clockwise to remove.

Don't forget to put anti sezie compound on the threads of new sensor.

Thank you, Danielsatur. (http://www.automotiveforums.com/vbulletin/member.php?u=559976)

Danielsatur (http://www.automotiveforums.com/vbulletin/member.php?u=559976),
Thanks for the info.

Dear moderator,
It's my third attempt to just say thanks to the author of the answers.

if too rusty try using aero-kroil or pb blaster let sit a little while