Pre wash the new gravel so you are not doing it after you take the fish out of the tank. Take out all the decorations and fish, scoop out the old gravel and put in the new gravel. Put the decorations back in, maybe let the water get less cloudy for a few minutes, and re-acclimate the fish into the tank. Keep a mesh bag of old gravel in the aquarium for a couple weeks to replenish the beneficial bacteria lost while changing the gravel. Test the water in the days after, and if you see any little spikes, you might have a mini cycle, but it will be fine if you do a couple extra water changes.
You do not need the undergravel filter anymore i dont think, but maybe leave it for a few weeks if you havent already, so the beneficial bacteria can get in the new filter.
Good luck and welcome to Aquarium advice