5.0 Nitrites is still pretty high. Keep in mind in a fully cycled tank it should be at 0. While cycling you should aim to keep this at least below 1.0

Dilution math isn't too hard. If you do a 50%( 1/2 the water) water change right now when its 5.0 then you will dilute by half (or 50%) and should have a reading of 2.5 Nitrites. A second 50% change should bring it down to 1.25. (Each time we are diluting by half, so unless your tap water has Nitrites you should be halving your reading.