Welcome aboard Tom.
There are circumstances that dictate whether one should do the repair from the inside or topsides. They both have their pros and cons, and challenges. The thread that Fred posted above is a good example of doing it from topsides, it's usually the easiest as there's no cabinetry, upholstery, etc. to remove and reinstall. Applying core and glass upside down and wetting it out can be frustrating as well, and there's the concern with making a mess inside the boat. Glass dust has a habit of getting into everything even when you think you've isolated the area properly.
The biggest challenges in doing it from above will be proper leveling and matching the surrounding non-stick pattern. If yours is just a roll-on pattern, then it's usually easier to simply roll out the entire deck so you can't tell the difference between the repaired area and the rest of the deck. Some patterned non-skids like the diamond pattern seen on some applications are a lot more difficult to reproduce in a repair and align/match with the surrounding areas. This is one of the reasons one might choose to repair from the inside instead.
Good luck, and let us know how things go. Will you do it or get someone else to?