a/c works left side but not right on 2000 BMW X5

a/c blows cold air from left dash vent but blows warmer as it goes to the passenger side.

Asked by for the 2000 BMW X5
A/C gas low
That worked, thanks...
How do you replace/fill A/C gas?? Do I have to go to a service station or can I 'try' to do it by myself. I really don't feel like getting screwed over by a $500 bill. Thanks!