Hmm.. config files are usually situated in /etc/squid/ if you have installed it from rpm or if installed from tar.gz package then it usually goto /usr/local/squid/etc/
To open them you can use any GUI text editor comes with Redhat like gedite, kedit, kate etc.
To open files in command line you can use vi.
for using vi do
vi <filename with path to file>
then press "insert key" and do editing ,
then press "esc key" to finish editing and to save .. type
:q <filename> it will save the file
to exit
: exit
To configure squid see
Configuring squid proxy server ..