Be careful not to update using the Sprint update process if you are rooted. Do you know the name and version of the Rom currently installed?
I recommend you do a little research online to familiarize yourself with the phone and software options available.
EVO 4G Android Development - xda-developers is a very good site with instructions and available Roms.
Be sure to create a backup of your current Rom (safety net in case of issues) before installing or updating.
Basic Rom installation/update from recovery -
(1) Wipe Data, Cache & Dalvik Cache, and System (optional but recommended the first time)
(2) Flash the ROM
(3) Flash GApps
(4) Reboot the system (first boot will take several minutes)