REMOVING THE 870 SAFETY:
Remove the two pins on the receiver which hold the trigger group.
On the trigger group locate a small pin behind the trigger guard on the inside of the group and located on the side of the group. You will need to drift the pin out.
BE CAREFUL since it is holding a small spring in retention.
The pin and the spring are EASILY LOST.....
Under the spring, which is in a small hole in the trigger group, there will be a small ball bearing, turn the trigger group over and let it drop into your hand. Put it with the spring and pin.
Push the safety out with your finger and slide in another safety.
Note the groves on the safety. This is where ball bearing will be seated, held in place by the spring which is held in place by the small pin.
Once you have the new safety in place, turn the trigger so the spring hole is up, put the ball in the spring hole, put the spring into the spring hole, depress the spring ( I find a ballpoint refill works well) BE CAREFUL since this is ususally where the spring pops out, hits you in the eye,scares hell out of you and goes to that dark place where all small things which are needed go to hide.
With the spring depressed, put the pin into the hole on the side of the trigger group and drift it into the other side which will place it over the spring and hold it in place.
CLICK the safety back and forth to make sure the ball bearing has seated into the notch on the safety.
Shuck the gun ( no shells loaded ) and see if it works.
Hope this helps.