2003 Subaru Legacy do I need refill

A/C works best when driving after car is off for awhile, when temps are around 80, less so well when 90's. A/c blows hot air when driving on an uphill drive (just regular no problem uphill), then kicks in again when driving downhill (cools well then). Is it getting kind of empty in there, is it something I can refill? There are plenty kits around. And if yes, where is the place i'd fill it in? Is it marked? (Haven't actually looked, but will tomorrow. So, other than hot air going uphill, there doesn't seem to be anything else. I am a highway driver mostly.
Thanks
Yvonne