Click on the Learning Experience Styling option located under the Design tab.
In the upper right hand corner will be a Downgrade to V2 button:
Click this button
A confirmation modal will appear to ensure that you do in fact want to proceed with the upgrade:
If you no longer need to do the reversion, click Cancel. If you'd still like to go back to V2, click the Downgrade button.
Your Learning Experience will now be set to V2 for all learners within your instance:
When you are again ready to proceed with the upgrade, you can follow these instructions to complete the process.