Knowing these values is of great use, however I will admit I would not know how to change, or otherwise manipulate them upon doing so, anyway. I apologize, but I am new to the world of hex editing and ROM hacking.
It's pretty much exactly like editing any other kind of file. You get a hex editor (I like XVI32) and you open the file. Typically the left side of the hex editor window displays the raw binary data, byte by byte, in the form of hexadecimal numbers
ranging from 00 to FF (that is, 00000000 to 11111111 in binary, or 0 to 255 in decimal). The right side shows the ASCII representation of the data, which will typically be complete gibberish.
If you want to change a value, you just select the value and type in a new one in the range of 00 to FF. It is important to only replace
values and not insert them, as inserting values changes the relative position of everything and can easily render the file unusable.
To be clear, only some
values represent data – program code also takes the form of hex values. Unfortunately, it's pretty hard to tell the difference at a glance. But a quick look at the .gbs files I have sitting around suggests that there are visible patterns in the hex values that probably represent data.