Hi, I'm Diane and I'm happy to help you today.
If you are using classic Outlook desktop software that is installed with Office desktop software, you should be able to switch back to classic Outlook from the Help menu, if not, you need to edit the registry. You can remove the New Outlook switch from it by editing the registry.
The registry keys you need to edit are in this article: https://www.slipstick.com/outlook/classic-outlook-not-new-outlook/
Let me know how it goes, I'm here to help you further if needed.
-- Diane
Disclaimer: slipstick.com is a site I run as a Microsoft MVP and the content is much more accurate and relevant than a general internet search. While the site is in English, there are translations options under the search box.