It may be hard to find the "Report Phishing" button when using the Outlook on the web (https://outlook.office.com)
STEP 1: Visit https://outlook.office.com
STEP 2: Open the email that you want to report
STEP 3: Open the Application menu
Look for the square icon on the top right corner of the email
STEP 4: Select the phishforward button
If using the Outlook mobile app you will follow similar steps