I have an upright Frigidaire freezer. It won't keep foods frozen. How can I eliminate causes of the problem? Can you get freon w/o a license? And are there proper steps to recharging the freezer?

If unit is newer than 1993 it has 134A in it which can be purchased at auto stores. However, without proper equipment and knowledge you will not want to attempt yourself. YOu will also have to understand the relationship of pressures and tempatures. Could be it's not even in need of refrigerant. I would start by cleaning the coils and fan first. Check to make sure the fan and compressor are running

I agree with Esquire1, clean the coils make sure that nothing blocks air flow. Make sure that the airflow inside the freezer is not blocked food packages. If it is a self defrost check the defrost timer.

I had a leak in the liquid line which I have replaced it, this is a 404a refrigrant system, I don't know what shoud be the pressures