Is my a/c overcharged and can I fix it?

My a/c is blowing warm air, so I bought a can of refrigerant to charge it.  
When I attached the gauge, the needle was almost in the red.  It was
between yellow and red, so I guess it was overcharged.  I watched a video
where they let some of the pressure out using the gauge with a towel
wrapped around it so it won't get on their skin.  So I did that. The problem is,
the needle on the gauge would go into the green area when I pulled the
trigger, but would go back up to yellow when I released the trigger.  So it
never released any of the extra.  Was I doing something wrong?  Is there a
way to release some of the extra pressure another way, and would this make
my air cold again?  Thanks for any and all replies.