Ahh cool, sorry it was just me that interpreted first post differently.
Changing fill nipple is a 2 min job, make sure you empty you tank first
When you say damaged, in what way, or is it just a leaking fill nipple ?
Take a look at this link.
Although the vid is showing you how to take the filter out of a CP fill nipple, it gives you an idea.
Also depends what Reg or fill nipple you have.
If you have a pure energy nipple, once you unscrew it off the reg, you will find a small piece cylinder shaped metal inside the back of the fill nipple which will have an o'ring attatched to it. This is call the check strut.
Once you take off any fill nipple, make sure threads are clean and in good condition (both inside reg and new nipple you are about to screw on).
Once you applied locktite ,screw in nipple, leave over night to dry with fill nipple facing down. just a precaution incase you apply too much loctite, not letting any excess run towards and inside of reg and bottle.