I think this might be a bit tricky. The web has been case sensitive for quite a while (read: since the beginning?). If you specifically want to have INDEX.HTML be synonymous with index.html, probably the easiest is to create a symbolic link. In the directory where index.html exists, just do:
ln -s index.html INDEX.HTML
If you want to have XYZ.HTML automatically mapped to xyz.html regardless of what XYZ is, then this would require some fancy apache directives. I would guess there is a way with all the fanciness that apache offers, but I'm not sure how. Do some searches for apache and case translation and see what you find.
Good luck!
-Danimal