With the Web Client scheduled to be removed entirely on the 1st October 2020, Microsoft are automatically transitioning environments to the Unified interface before this date, you may be surprised to see how early they have scheduled the transition if you were expecting to have until October!
The Transition Portal can be found at https://runone.powerappsportals.com, you will need to sign in with admin credentials, and ideally a Global or Service admin account as these are required in order to change the transition dates.
To submit an exception request to either bring the date forward or push it out, you will need to be a Global or Service admin. You can only push out the date up to 30 days, however you can make up to 2 requests to get another 30 days.
For more information about transitioning to the Unified interface, you can check out the Transition to Unified Interface section on Microsoft Docs.