1. On top the menu bar click on outlook and a drop-down menu will appear.
2. If there is a check mark beside legacy Outlook, you are using Legacy Outlook. (and if there is no check mark, you're using New Outlook)
To shift to new outlook select the Legacy Outlook option, same as in the above picture and you will get a prompt to shift to new outlook.
Repeat the same steps in new Outlook to shift back to Legacy Outlook.