The OnePlus 6 and 6T are currently in the process of receiving their Android 10 update. However, it's best that you don't install it just yet, given that it does more harm than good. You can read more about the problems it triggers here. If you have already installed the update and your experience is less than optimal, here's how you go about downgrading it to Android 9 Pie.
How to downgrade to Android 9 Pie from Android 10 on the OnePlus 6 and OnePlus 6T
Before you proceed, note that this process triggers a complete wipe of the on-device data, so it's best that you make backups. If possible, leave the device plugged into a charger while during the process. If that isn't an option, ensure that your device has at least 50% battery. You do not want your phone battery dying midway, as it could potentially brick your device.
Step 2: Copy the .zip file on to your phone and place it at a handy location.
Step 3: Navigate to Settings -> System -> System Updates. Click the top-right icon and select Local upgrade and navigate to the corresponding installation package that you just copied.
Step 4: Confirm that you want to go ahead with the procedure in the next step. You'll be prompted to enter your PIN/Password/Pattern at this point.
Step 5: Wait for the system upgrade to finish installing. Your device should reboot after the process is complete and you'll have to set up your phone from scratch.
And that's it, you should now be on stable Android 9 Pie. Follow the same steps (but with the Android 10 package instead of the Android 9) to get back to Android 10, once OnePlus has fixed all the underlying issues with the software.