how to downgrade to oreo

Please tell us some basic information before asking for help:
- Model Name: ze620kl
- Firmware Version: ww_16.0615.2005.130
- Rooted or not: not
- Frequency of Occurrence: -
- APP Name & APP Version (If your issue relates to the app):
In addition to information above, please also provide as much details as you can, e.g., using scenario, what troubleshooting you've already done, screenshot, etc.
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how to downgrade form pie to oreo because i find some problem with pie like camera whatsapp and instagram so dark but on pixel master good enough
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Tell me how can I downgrade from android 9 to android 8.1
I have already downloaded the stock firmware from support website of android 8.1. please help me.
I have downloaded the stock firmware android 8.1. tell me how to downgrade it from android 9 to android 8.1