you need to download the firmware from my link below
You can choose LG-D415(L90)_Android_KK_D415V10e
Then follow this guide to install it
To Root the phone when needed as per the instructions you will be following, you can use this guide
Remember that downgrading or anything to do with the firmware can results in a bricked phone (completely unusable) and will results in a voided warranty. So proceed with caution.
Also ensure that any data is backed up as this will remove all data from the phone.
If you would like any additional information or assistance, please do not hesitate to let me know.
And if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.