How do you change the actual metal key in the remote transponder 1997 E300 Mercedes?

easy, pop out the battery cover and just pull it apart. When you reinsert the actual metal key you'll have to wind the spring 1 revolution to load it to spring.

The spring will have a small leg sticking out, place the leg into the plastic housing whee there is a slit. The wind the spring and snap the two plastic pieces together.