#1. Lowering nitrites is the same as lowering nitrates, Water changes. Other than that, like James said, it is going to take time. It sounds like the tank is new and you are just now seeing the nitrites, This is perfectly normal and they will go away. If the tank is cycling, Water changes are only going to slow down the cycle. The fish you have will either suffer the nitrites, or you can see if the LFS
will take them back until the tank has cycled.
#2. If the tank is not new, and you are seeing a nitrite spike of that magnitude, then you have got something in the tank creating ammonia which is being broken down into the nitrites. Not a good thing in a seasoned tank..
In either case, the tank is going to have to settle, and no matter what you do, it is going to take time. So as stated before, you have a choice.
Do water changes to keep the toxicity down and prolong the cycle and pray the fish make it through it, or, take the fish back to the LFS
and don't do a water change and let the tank cycle.