Caution: This is only to unlock the oem of your mobile 
            phone and should be done only if you have android 
            12 or above in your device
Enable Developer Options:
3.Tap on Build number 7 times to enable developer options
- Head to developer options on your phone and turn the 
 on advanced reboot and USB debugging.
  
- Connect your phone to your PC with a USB cable. 
7.Once you've backed up all of your data, you'll need to 
   download an Android factory image of the version you want 
   to go back to. You can find this at the manufacturers 
   website if not then best place to find it is XDA forums 

- 
Download the SDK platform tools if you are using windows then, - Extract the zip file to the desired location.
 
- Navigate to the platform tools folder and type cmd in the search text-box
- Then type
adb devices
in the command prompt. You will get a string with the device being connected to your pc
- Type
adb reboot bootloader
in the command prompt.
- Now you learnt how to set your phone to fastboot mode. 
- Press the power button and boot your device. 
- Search for your local updater apk (for oneplus it is 
 oplocal updater
- Caution: remember to do this in fastboot mode 
 From the stock rom you downloaded, take the .img file
 and type the following command in the same directory
 
fastboot flash bootloader [bootloader file name].img
- Downgrade to the desired version by selecting it in the apk installer.
Now you are good to go your mobile phone will be downgraded to the one you like.
 









 
    
Top comments (1)
I think that the MSM tool is an alternative way to downgrade, but this is easier i think