Wow!
An hex editor is an hexadecimal editor. It's a tool that can edit bytes of anything. You just need to open your hex program, open a file, and edit by yourself.
Ok, first, you'll surely see a lot of numbers everywhere. These are called bytes (quite evident, no?)
Now, look at "l2level.txt", you'll see the coordinates where are stocked the infos of the uncompressed levels. For example: from byte 0 to byte 3, there's "FORM". From byte 4 to byte 7, it's the file size. And so on... In your hex editor, you should see the coordinates of the bytes somewhere.
In hexadecimal, values are from 00 (0) to FF (255) and can even use multiple bytes like 00 00 (0) to FF FF (65535) or FF FF FF (16 777 215).