ok so all those modded songs on guitar hero are modded via byte injection. right? so how does it work? and couldn't this work on other things? like system update files? I know that these files are encrypted and just look like a jumble of random numbers letters and symbols, but if you got lucky and found the spot that tells it what version it is, couldn't you make it think that an older update is a newer update?

before anyone shoots that idea completely down, someone should at least give me an explanation on how byte injection works.

Some files are signed, some others aren't. If you change a single bit inside a signed file game won't work anymore.
A signed file has a signature in a file. Only a private key (no one knows it outside console manufacturer vault) can create it, but devices have a public key (we know it but it doesn't help to find private key quickly) that allows to check that the signature is valid. 'Brute force' is the common term for any attempt to find such private key and usually requires centuries or more to obtain it by numerous tries.
Google 'rsa' and 'sha-1' for details about involved mathematics.

so the song files in guitar hero aren't signed then?

Nope, the actual song data isn't signed. A few basic bits of information about the song are stored seperately and signed, though, which is why you can't change the displayed song titles, and why you can't replace a song with a song that's longer.