How do I change the return springs for the DCN690 and DCN692 Framing Nailers?

You should be able to do this easily, using only the hex key supplied with the nailer. You will need the DCN6901-XJ kit which contains - return springs, foam dampeners, washers, rail end caps, shoulder screws and the thread adhesive required to make the spring change.
You would use the kit to replace return springs when they have reached the end of their life.  You would notice that the springs require replacement especially when the driver regularly stops returning after driving a nail.
The video below will show you how to change the return springs.