you should backup ur current rom at first.
install any custom recovery, I suggest clockwork mod recovery, . u can flash it via rom manager.
after that download the rom u want 2 install
then
** reboot to recovery
** wipe cache ,not important.
** wipe dalvik cache
** select install zip from sdcard and
select the pre downloaded rom file
** reboot
wait for the reboot to complete and u r done.