I already used YY-CHR to edit GFX files of Super Mario World sprites, but I don't know how to edit levels, because when I load any Super Nintendo ROM, the graphics appear illegible, even if I use the SNES graphic mode.
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Unfortunately, this a bit out of scope both for NEXXT, and for my own personal expertise.
The only further advice i can give would be join the SMW central community, and to browse through its resources: https://www.smwcentral.net/