If you dont want to open a can of worms and you want to get the best result,then you go to a garage where they know what they are doing and in the text that follows you will find the answer,why not to do it yourself at home with a can full of refrigerant like this
http://0.tqn.com/d/autorepair/1/0/Z/5/-/-/ac-intro.jpg
This is direct info from volvo vida.
Refrigerant, draining
Note! The recycling unit must have an oil separator. The oil separator must be of a type that allows the amount of run out oil to be measured.
Note! The ambient temperature must be above 20°C(68 °F) when draining the system.
When draining refrigerant, the recycling unit can be attached to the service valve for both the high pressure side (HP) (1) and the low pressure side (LP) (2) at the same time. This is the quickest method.
Check the amount of oil (if any) that has run out when draining the refrigerant.
Vacuum pumping
Low pressure side (LP) service valve (2) is concealed under the plastic cover above the wiper well.
During vacuum pumping, the vacuum pump (recycling station) can be connected to the service valves for both the high pressure (HP) (1) and low pressure (LP) (2) sides.
Operate the vacuum pump for at least 50 minutes.
The vacuum must be checked after vacuum pumping.
Fill refrigerant directly after vacuum pumping. See below.
Note! The vacuum test is not a substitute for leak tracing.
Refrigerant, filling
Only fill refrigerant on the high pressure side's (HP) (1) service valve (TXV system). A specification decal in the engine compartment indicates how much refrigerant the A/C system holds.
Note! The most common causes of failed filling of refrigerant is incorrect vacuum pumping or because the pressure in the filling unit is not high enough, or a combination of these two.
Note! The filling unit should preferably have a heating unit for refrigerant. This is so that the pressure in the filling unit is higher than in the system that is to be filled.
