You can use these to, well, edit your NES ROM (not that we necessarily condone that). There are several from which to choose.
Widely considered to be the finest editor around, Disch, formerly of OPGaming way back in the day, has created an awesome program to edit spells and spell level points, treasure chest contents, maps, and a lot more. We don't even bother to offer other editors. Check for updated versions at Disch's Homepage at Zophar.