What is the best way to get out an access plug that is located at the tops and side of the P trap drain? It is a brass plug that has a square head that is about .75 x .75. The plug has never been out. I sprayed it with penetrant spray. Not enough room to get a pipe wrench on it. I tried a regular open end wrench and it does not loosen.

Leave the plug alone and buy a regular new p trap

Are you talking about a trap that is a floor drain or a trap that is under a sink?
If it is a floor drain trap you might try removing all of the water from around the plug and then heating the plug, sometimes this will break the bond and let you unscrew the plug.
If this is not in concrete and under a sink, do as Doug said and replace the trap.

What is the best way to get out an access plug that is located at the tops and side of the P trap drain? It is a brass plug that has a square head that is about .75 x .75. The plug has never been out. I sprayed it with penetrant spray. Not enough room to get a pipe wrench on it. I tried a regular open end wrench and it does not loosen.
This sounds like a brass clean out plug in a cast iron floor drain. If the drain's clogged you can snake the trap and branch without having to remove the plug by going down the trap. Good luck, Tom

What is the best way to get out an access plug that is located at the tops and side of the P trap drain? It is a brass plug that has a square head that is about .75 x .75. The plug has never been out. I sprayed it with penetrant spray. Not enough room to get a pipe wrench on it. I tried a regular open end wrench and it does not loosen.

Wayno2, when someone gives you an answer in one of their post they would appreciate it if you gave them some kind of an answer whether it helped or not, that way we can lay our heads on our pillow at night and have the fulfilled feeling that we accomplished something this day.
Just kidding but we would l8ike to know if our answers did anything for you.

I agree with letmetellu heat around it but be careful don't burn the place down.

I ended up chiseling out the brass plug and snaking the line. The drain now works. I replaced the brass plug with a rubber one.

Thanks for the reply. At least we now know that your problem is solved for the time being.