I would start with daily 25% water changes so as to avoid shocking the fish with different water parameters.
Don't forget to add the appropriate amount of water conditioner that treats either chlorine or chloramine, depending on the water source.
Do NOT replace the filter media at this time. You can used dechlorinated tap water to rinse any canister filter media to remove the filth and IF the filter uses fine media pads, it's OK to replace those.
I would wait and not remove the gravel at this time for at least 2 weeks but you could use a gravel siphon to remove any dirt and debris that is trapped in the gravel. Do this in sections until you've removed 25% of the tank water and then add fresh water and dechlorinate it. Consider this your 25% water change for one day.
Are you familiar with maintaining an aquarium? Not sure of your level of expertise.