Install Android 2.3 Gingerbread on HTC EVO 4G
Here are the steps you must follow to install Android 2.3 on the HTC EVO 4G.
1. Download the Evervolv v1.0.0 a9 Android 2.3 Gingerbread ROM for your HTC EVO 4G. You will need this ROM to install Android 2.3 on your phone. It has been compiled from the Android 2.3 AOSP source.
You can download it here - Evervolv v1.0.0a9 - Gingerbread
2. Install the ROM Manager application. Go to Android Market and search for ROM Manager. Install the Clockwork Mod ROM Manager app. Remember that you need to have root access on your HTC EVO 4G to install it.
3. Start ROM Manager and click on the Flash Clockwork Mod Recovery option to create a recovery image for your phone. After this, click on Backup Current ROM to backup your data and your current ROM. The backup will be saved in your SD card.
4. Now connect your phone to your PC, and copy that downloaded ROM to your SD card.
5. Open the ROM Manager application again, and click on Install ROM from SD card. Select the ROM you just copied and press OK to continue the installation. In a few minutes, the installation should be complete and your HTC EVO 4G will be running Android 2.3.
6. If you want to go back to your backup (original ROM), start ROM Manager and restore from your backup file on the SD card.
Congratulations, you have successfully installed Android 2.3 Gingerbread on the HTC EVO 4G.
ROM Source - XDA