Why do you want to replace the ejector depressor plunger spring? The ejector is held in the gun by a pin and held in position by the magazine. The only function of this spring is to depress the ejector when the magazine is out so the gun cannot be fired without the magazine. If you are having problems with ejection this isn't the problem.
If you insist I will tell you how to remove the sight, but there is a real hazard of breaking the rear sight spring when you do this. In the mid-70s I maintained several hundred M39-2s as my department's armorer, and I did have several break when removing the sight. I had more than one new sight break when installing it for the first time as a replacement part. Lots of luck finding a replacement!
Look at the top of the slide and you will see a roughly semi-circular hole at the front of the sight spring. Put a small punch into the hole and use it to pry the front end of the spring up until it clears the slide recess. While holding the spring up rotate the sight 90 degrees either way and the sight then lifts out of the slide. As they say, assemble in reverse order.
This spring, and it's plunger, serve no purpose other than to push the ejector down when the magazine is removed so the gun cannot be fired. The gun will function perfectly with both the spring and plunger, or just the spring, removed. Unless there is a compelling reason to change or replace this spring leave it alone. If you break the sight spring don't say you weren't warned.
