You need to do a couple water changes today and a couple tomorrow because it is at a very dangerous level.
If your water parameters for pH etc. are similar to your tap water, I would do a 80-90% water change.
If it isn't too far off, I would do 2 water changes, 40%, then in an hour I would do a 70%. That is to get the 2 types of water, tank and tap, more similar.
If it is not very similar between the 2 - water changes should be more like 30%, another 30% and then even another 30%.
After the option which fits your water, then...
Check it out with a test, and then tomorrow, do more water changes until the nitrIte is 0.