First let me start by stressing something important! Verify the integrity of the download by checking the file against its md5 checksum that I provided for each file. You can use a free software such as MD5 Checker to do the job. Or if you'd like to check on your phone without a pc, download MD5 Checker for android from the play store. If for whatever reason your download gets corrupted, using a corrupted file can brick your phone! So use an md5 checker to verify the integrity of the file, it's quick and easy and can keep your phone safe!
1. Download File. Check md5 for a match
2. Place the file on the root of your sd card (not in any folders).
3. Rename the file to PG86IMG.zip
4. Reboot into bootloader.
5. When asked to update, say yes by hitting volume up.
6. When it's all finished, hit power button to reboot.
7. *Important! After booting up, immediately rename or delete the file.
*The presence of the file on your sd card will prevent you from entering recovery in an emergency situation. It's nothing to panic about, but if you get caught in a bootloop, you will have to manually remove the file using a pc and card reader or another phone. That said, to avoid bootloops and being stuck with no way to boot up and no way to enter recovery, plan ahead. If you are on a rom that requires hboot 1.58 and you flash hboot 1.5, you will hit a bootloop and not be able to access recovery. You would then need to remove the file or flash a compatible rom through fastboot connected to a pc.
Okay. Now with instructions and warnings out of the way, I will give you my personal preference on how to flash a different hboot that will be incompatible with your current rom without getting a bootloop and having to use a pc to fix the situation.
Follow steps 1-3 above. Stop! Now, instead of rebooting into the bootloader, we are going to bypass the bootloader by rebooting directly into recovery. Some roms have this feature built in. I recommend downloading something like ROM TOOLBOX from the play store. It's a great app to have if you are rooted. Rom Toolbox has a built in rebooter menu (with widgets) that allows this function. So once in recovery, wipe and flash the rom normally. Once finished, continue to steps 4-7.
This will allow you to do everything completely from your phone and avoid getting stuck. Remember, this is just for when you are changing hboots that will be incompatible with the rom you are already using (ie: ICS to GB). Just following steps 1-7 normally is fine for just flashing radios or switching between different GB hboots etc.
