Rename the file to ‘update.zip’ (make sure the file extension isn’t doubled [E.g. 'update.zip.zip'])
Power down the handset
Hold down the up position on the volume rocker while powering up the phone to enter the phone’s bootloader mode
Use the volume rocker to navigate to ‘RECOVERY’ and select with the ‘power on/off/standby’ key to enter recovery mode
Select ‘install update.zip’
Wait for installation to complete and the phone to reboot then enjoy Ice Cream Sandwich
It’s worth nothing that updating via this method will afford you the same results as those that wait for the file to be pushed by Sprint. This is the official software release all users will receive. If you have any apprehensions it may be best to wait until Sprint officially launches the rollout of ICS.