How do you replace a radiator of 1991 BMW 525i?

Disconnect the hoses connected to the radiator and drain the system. (The water will come out by itself :) )

Then you just take the old radiator out and put the new one in.

Connect the hoses, fill the system up with water (you might want to add some antifreeze) and don't forget to air the system.

There is a little screw on the radiator that you have to loosen, and when water is coming out from the screw, you tighten it again.

Good luck!