Epoxy Over Paint
You can apply epoxy over garage floor paint or over another coat of epoxy without removing it, but you have to prep the surface. Start by renting a floor buffer with a 60-grit sanding screen. Get extra screens and return the unused ones for credit. Run the buffer over the floor to remove loose paint and scuff the surface to create a texture for the epoxy to stick to. Sweep the floor clean. Then, just as important, clean the floor with a cleaner/degreaser, available at home centers. The cleaner/degreaser will break the molecular bond of the grease so it’ll come off the floor. Epoxy won’t stick to oil- or grease-contaminated floors. Let the floor dry for a full day, then apply the coat of epoxy.
If your garage needs a new floor, here's a collection of options.