1. Install the ROM Manager on your HTC Jetstream. Get it from Android Marketplace Location, then install the clockwork mod recovery.
2. Connect your HTC Jetstream to your PC employing the USB information cable. Copy the “HTC Jetstream_4.0 ICS/Ice Cream Sandwich.zip” file you downloaded to the SD memory card.
3. Restart your device in the recovery mode using ROM Manager. As soon as in Recovery Mode, go to ROM Manager and select backup / restore to back-up your information.
4. From the Recovery Menu do a total data erase (including Delvik Cache). When completed, go back to Recovery Principal Menu and choose Flash ROM from SD Card. Navigate the location you copied the ”HTC Jetstream_4.0 ICS/Ice Cream Sandwich.zip” file and follow the directions for installing Android 4.0 ICS/Ice Cream Sandwich on your HTC Jetstream. As soon as the installation is over, just reboot your HTC Jetstream device, and here you go, you’ve just upgraded it to the new Android 4.0 ICS/Ice Cream Sandwich
Note : Here Is Link To Download the ROM/Zip File :