Water changes. Your tank isn't cycled (what is your ammonia reading). The only way to lower nitrite or ammonia until the tank finishes cycling is water changes. Do a 50% WC immediately and then another one in 2 hours and another in 2 more hours. That will probably still leave the nitrite at .75ppm and you need to get the ammonia and nitrite below .25ppm. After that do 20 to 30 percent changes as needed to keep the ammonia and nitrite below .25 ppm.
What size is the tank and what/how many fish are in it?