Different people tend to have a different range they feel is acceptable. Also, some species of fish are more tolerant of NO3 than others. For a non-planted tank, I would keep levels as low as possible, preferably <20ppm. With sensitive fish, <10ppm.
In a planted tank, I'd keep the NO3 between 5-10ppm.
When NO3 is high, water changes are the best way to bring it down. If it's been a long time since the last water change, start slowly, with small amounts to avoid shocking the fish. Continue changing water daily, increasing the percentage each day, until levels are where you want them. After that, you can use NO3 levels as a monitor - if your acceptable level is 20ppm, when NO3 rises above 20ppm do a water change to bring it back down below that level.