JeebusFright t1_j73ah2e wrote

Remove all the old sealant, clean up the surfaces as best you can, use a sealant removing product if need be. Reapply new sealant. Don't cheap out on the stuff, and if you need a plain white one, consider a mould resistant version. Applying it is a dark art, as others have suggested, watch some YouTube videos and then maybe practice on a scrap of wood to get a feel. Good luck!