I agree, donot empty the tank, just return the fish and keep a source of ammonia going- this came by the way of fish poo, as you're returning the fish you can buy the bottled stuff, DONOT use live fish that you don't mind dying- this is such bad advice and wrong on so many levels.
There is no need to scrub anything down, there is no need to clean out the filter or change the filter insides, the good bacteria builds up on everything inside the tank and this needs to be maintained through adding ammonia which in turn turns into good bacteria thus calling it a cycle. You can read about it on the link I posted.
You will need to buy a testing kit, and once everything is all good and the water is habitable we will help you with your stock. Hopefully we can get your tank up and running safely and properly.
so first thing is returning the fish and picking up a bottled product to help with the cycling process.
Come on line and ask us for help when you have it.