To downgrade a Dynamics 365 version, first log into https://portal.office.com and go to the Dynamics 365 Admin Center:
Select your environment and click Edit. Set the environment to Sandbox mode.
Next, click on Reset:
Select the target version to reset to:
Click Yes to Reset:
Go back to the Admin Center home page. You will see the instance is being set up and the state is “preparing instance”:
If you access the organization, you may see the message that your user does not exist. Wait a few minutes.
You will then see:
In the Admin page, set the type to Production:
That’s it. Check the About page to confirm the version has changed.