head to your reposity directory, which is probably something like /etc/yum/repos.d
(I'm 99,9% positive it's under /etc/ and called something like repos.d
under something like yum
), where you will see several files. These files describe your reposities (the addresses), so if you open them up one by one and see what's inside, you'll get the thing: just create a new file there, which is of the same form as the others are, but put your own URL(s) in the file.