I peed.
You would have to recover the refrigerant and weigh it.
0Psia
This is most definitely not a DIY task. To do it properly, you need to first test the purity of the refrigerant, which takes special equipment to do, as does actual recovery and storage of the refrigerant.
Use a vacuum pump that pulls it into a tank.
Take it to a shop that does ac work and have them recover, evacuate and recharge the system but the question I have is why do you want to change the refrigerant?
The only legal and safe way to do it is to have a shop recover the refrigerant with an RRR (Recover, Recycle, Recharge) machine. Try your local community college if they offer a vocational program for mechanics... they might even pay you for the refrigerant they recover.
The EPA.
You recover the refrigerant with an AC recovery/recycle machine.
In order to make it easier to recover refrigerant.
Short answer is that you don't. If you need to do this, then you have a certified shop recover and store it.
No. You absolutely cannot mix any refrigerants. You should completely recover all the old refrigerant and charge the system, with the new refrigerant, using the superheat method.