If you want to downgrade the version of your mobile phone system, I suggest you back up your mobile phone data (phone number, SMS, photos, etc. ).
Pay attention to the official account of "ColorOS Upgrade Assistant" WeChat, and click the downgrade tool in the lower right corner of the menu bar for self-downgrade.
If you can't downgrade yourself, you can bring your mobile phone, purchase invoice and warranty card to the official service center of OPPO, and let the engineer handle it for you.