Ok. The first thing you need to do is take down the teak trim piece above the porthole, it's attached to the fiber glass with 4-5 screws. Then you can reach up and feel the two bolts that are on the flat part of the deck. Mark the fiberglass lip so you know where they are. Then you cut the fiberglass so you can access those bolts. The two that go in the deck have fender washers, lock washers and nuts on the bottom. The but is 11mm.
The two that go into the tow rail are threaded into an aluminum plates bedded into the towrail. I use a #2 Phillips head that goes in a ratchet to break these free. Then a large screw driver to get it the rest of the way out.
Now you will probably want to replace the bolts. Check them carefully. I found most had a slight bend to them. Eventually I gave up checking and just replaced them all. I replaced the fender washers and went to nyloc nuts too.
Now you are going to probably find rotted wood core on the two that go through the deck. Go to Marine How To article on bedding with butyl tape. He has a great write up on potting holes for deck hardware. I did that to repair the small amount of rott and prevent future rot. I did it on every stanchion regardless of finding rot. I also counter sunk the holes for butyl like he discribes in the article.
I used butyl tape from Maine Sail. The bottom ones are done just like he shows.
The ones in the tow rail are a little different. I found the best method was to actually put the butyl on the tow rail, not the stanchion base. Leave a gap between the butyl and the holes. Soften the butyl around the holes with a little bit of mineral spirits on a qtip. Then put a thin wrap of butyle on the head of the screw and insert the screw into the base. Now put another thin wrap around the screw about 1/8 inch down from the stanchion. Soften that with mineral spirits as well. Now when you screw it in butyl should fill the gap you left.
This stanchion and the opposite one are the most often damaged by docking and may be bent slightly inward. Look before you start. I put a 1/4 piece if starboard under the part that sits on the deck as a spacer to straighten it out. A better option would be a new stanchion but priorities.
Good luck.