How Do I change unchangeable tilesets!!!!!!!!!!!!!

I remember playing Hero's Quest and they have tilesets all over the place that you would have to have completely new maps for, and I would love to get a clear explanation on how to do this!! It opens up so much possibilities for maps I can't believe I couldn't find anything in the Tutorials, map development, or this forum. I searched for about an hour and downloaded Zepir's editor because I think it's the tool I need but I don't know how to use it, and the version I got from this site seems pretty outdated I'm afraid it won't even be compatible. I've only seen a mix of unchangeable tilesets in Hero's Quest but I'm sure the guy who made it can't be the only one who knows how to do this! Why hasn't there been anything about this before anyways?

If you want to change an unchangable one you have to import a tileset texture ranaming the one that is not changable.

You can download WE Unlimited here

It can add up to 16 different tiles to any map.
When changing the tiles you can still edit the map with other editors such as UMSWE or the ordinary WE.

See the tutorials on the homepage for further info.

I've tooled around with the World Editor Unlimited now and I can get new tilesets but I can only use th original cliff tiles for the map (dirt and grass). Basically, I need to be able to cliff square tiles from cityscape so I can put places in my map that look like the hero is inside a building. It's possible to have a map that has inside and outside, right? Hero's Quest doesn't work anymore so I couldn't check if that map does it or not, but is everyone really limited to a map that's purely indoors or outside?!

You're right. Additional cliff-tiles only work with the game patch.
But if you don't want that there's no way.

Which Game Patch would that be? And where can I get it?

It comes with WE Unlimited (look in the install dir of WEU)
Also if you don't know how to use it to have more than 2 cliff tiles you should check out the WEU homepage at umswe.wc3campaigns.com/weu where you'll find the documentation