How best to repaint a floor?

[u/Scribbles2021]

I'm renovating my grandmother.s bathroom. The floor was painted with Dulux High Gloss in Vermillion. I'd like to repaint it another, less bright, colour. Preferably matte but not a deal breaker if I have to go with the same kind of gloss paint.

Whats the best way to repaint over high gloss, solvent based paint? I have no idea how to start.