Some users may find that Outlook (New) does not have the features that they require, and they may need to switch back to Outlook Classic. This can be done by multiple methods depending on how Outlook (New) was launched.

First, in Outlook (New), check if the following option is in the top right:

If so, toggle it off, and you should be returned to Outlook (Classic)

If this does not appear, try opening Outlook (classic) from the start menu. This can be done as shown in the following screenshot:

If this opens successfully, open Settings->Apps->Default apps, Search for Outlook, and select the highlighted one

Use the icon highlighted as a reference - not the New message, as this is referring to whether the app has been opened or recently updated, not it being Outlook (New).

Then select the Outlook with that icon for each filetype in the list.