Go to Microsoft 365 mail on the web (you can use https://outlook.office.com/mail to get there directly).
At the top of the page is the search bar. Type in your terms, and you’ll see the results (with a nice new preview of recent items as well).
If you want to get granular, click on the 3 lines on the far right of the search bar, and you’ll be able to fine-tune your results to include the subject, keywords, the date range, and if the messages you want have attachments.
Like this? Some more Office tips here: Microsoft Office – Macnamara ICT