Microsoft has been lurking about the idea of placing the Outlook navigation bar to the left side for quite a while and has finally pulled the trigger lately. If you are one of those who, I honestly have lost count of how many, genuinely hate this new look, here is the solution for you.
Go to File > Options, switch to the Advanced tab and uncheck the option called Show Apps in Outlook.
Click OK to save the setting. Restart Outlook and you will be back to classic mode.
There is also a registry hack you can use to trick Outlook as well.
Press Win + R, type Regedit and press Enter to open the Registry Editor.
Navigate to the following key,
And set the following value to false.
Obviously, the native setting option is always recommended but the registry option does give SysAdmins a bit more options to manage a large group of computers. You can export the key and import them as you wish. You can also use the PowerShell script to remotely push out the change as well.