The Outlook desktop app has two main looks. The Old Look and the New Look.
As users we can use whichever seems a better fit for our needs, but to be noted some changes that take place on your Microsoft Account on behalf of your organisation, are showing up quicker using the old look.
To change between the two looks of Outlook, follow the below steps:
Change from the old Outlook to the new:
1. On the top of you window, click on the button next to "New Outlook", and turn it on
2. After enabling the New Outlook, you might get an alert message, informing you that some of your accounts cannot migrate, and you will have to add them later. Note down those accounts to add them manually afterwards, and click on "Open New Outlook".
3. After this procedure, Outlook will close for a couple of seconds and re-open with the new look.
Revert back to the Old Outlook:
1. On the top of you window, click on the button next to "New Outlook", and turn it off
2. After disabling the New Outlook, you will get an alert message, asking you if you want to switch back to the old look. Click on "Switch Back" and wait.
3. After this procedure, Outlook will close for a couple of seconds and re-open with the old look.