The very first thing that you need to do is have the system properly re-charged as you're probably low on 134a ("freon"). The home kits may not be a good thing because you have no way to know the current charge and you can easily put in too much, equally as bad as not enough.
A 134a AC machine in a shop recovers the existing 134a and weighs it thus the tech knows how much (or little) 134a was previously in there. Then the correct charge is installed, 1.63 lbs (no more, no less). If not done before, Dye should also be added so that if and when it leaks out slowly, it can be easily spotted the next time.
Once you know that the system has the correct charge of 134a, if it's still not functioning properly, that will need further diagnosis BUT the odds are good that you're just low on freon as that's the most common problem with 850s.