Better, to go to a shop. If the unit needs recharged it has a leak and recharging is only a temp fix. If it is a older unit it may well be set up for R-12 freon and that is very very hard to come by since it's been banned. There are a fair number of special tools to do it right. If it is a later unit it will have R-134A in it and that you can buy it at napa. (tools still needed) There should be a tag on the unit under the cover saying what freon is being used. Hope this helps, Tony, Joy and Cool Paw Luke
All roof airs Ive owned DO NOT have any service port whatsoever (like an auto system with one tap on high pressure and one on low pressure suction side) whereby you can check Freon level plus add some if necessary. HOWEVER (1) I have had techs tap and add a service port thereby enabling them to check and add freon if necessary and some have been repaired that way and were fine. HOWEVER (2) Since they are closed systems if they are low on freon theres often a leak and if they are already old Id suggest a new unit versus fixing an old one. Its worth a try, if the compressor is still okay and it was a slowwwwwwww leak you might be able to tap and add a service port and recharge it but thats gonna cost $100 and then if it leaks again youre out that money which could have been better spent on a new unit.