Changing the safety is easy with a little care.
Assuming you have a RIGHT-HANDED 870 and have a new right-hand safety available, here's how to do the job:
Remove the trigger group.
Un-cock and gently lower the hammer.
Near the rear of the trigger group is a large, hollow bushing. On one end is a spring clip to hold it in place.
Push this bushing out from the side without the clip.
This will allow the trigger to move forward.
At the extreme rear of the trigger group you'll see a small silver cross-pin.
Directly on top of the trigger group above the silver pin is a hole.
With your finger over the hole to catch the spring, use a small punch to push out the silver pin, from LEFT to RIGHT.
With the pin out, remove your finger from the hole and carefully remove the small coil spring.
Under the spring is a tiny silver ball. This may or may not come out if you turn the trigger group over. If it doesn't, leave it in place.
The safety will now drop out.
Insert the new safety, being sure the RED band is on the LEFT side of the trigger group.
Insert the ball if it came out, then insert the coil spring.
Use a small screwdriver or punch to push the spring down while holding the safety in place. (As an aid, you can use a strip of tape to help hold the safety).
With the spring pressed down, insert the silver pin TAPERED END FIRST.
Make sure it passes over the top of the spring, locking it in place.Press the pin fully into the trigger guard.
Re-insert the large hollow bushing, keeping the spring clip on the same side as it was.
Test the safety to insure it operates properly and prevents the trigger from moving.
This actually takes almost as long to read as actually doing the job.