2 Quad EL34 sets for Dynaco ST-70

I am new to tube amp and I really need your help. I have one current quad Russian EL84 running in ST70. My friend gave me another brand new quad Mullard EL34 Russian tubes for me to test sound. What is the best way to remove old quad tubes out and replace new quad tubes? If I do not like new one, I also can put my old tubes back.