Assuming you have fastboot installed on your computer (or for Win/Mac you could use the version bundled with the install package):
- Download the manual install package for the version of the OS you’re using to your computer (match the version number to what you have installed - if its an older version, you can find the link here).
- Extract the install package.
- Find the path to boot partition image file in the extracted files (usualy
boot.imgunderimages. - Connect the phone to the computer with a USB data cable and start the phone in fastboot mode.
- On the computer run
fastboot flash boot [imagefile]in a terminal, where [imagefile] is the path to the image file you found in step 3. On linux systems you may need to usesudofor this step. - Reboot the phone.