Loosen turnbuckles, one on each side
disengage chainplate from deck, clean out hole in deck, caulk with silicone or sealant of your choice, reinstall.Really, I know you have lots of new questions with your new-to-you boat, and you may take advantage of the archives on this site, and the great text and stories on most other boating sites, like BoatUS and other associations websites, which will all give you detailed steps for this work.Each boat's chainplate connections are a bit different down below (i.e., the C30s hit a bulkhead, the C34s go to tie down straps, don't know what yours do), but the above deck stuff is much all the same.Take one apart and see how it works.