Preparations:
Download the firmware and copy it to the root directory of you phone storage.
Make sure you got enough battery left ≥ 30% would be better.
Here we begins:
Turn off the phone.
Hold down Power key + Volume down key at the same time, when OPPO logo appears release your fingers, after a few seconds you'll enter recovery mode.
Select the Language. (OPPO recovery support touchscreen operation, if your recovery is the early version which doesn't support touchscreen operation you can use Volume up/down keys to navigate and use Power key to confirm).
To make a clean install please select "Wipe data and cache", press "Yes" when it's done.
Select "Install from sd"
Tap the firmware you've downloaded and press "Yes", the updating will finish in a few minutes.
When the update is finished tap "Reboot" to finish updating.