Step 1: Login into portal.office365.com

Sign in with your Login ID and Password.

Continue…

Step 2: When entering account click on Admin as shown into the picture.

Step 3: In the Office 365 admin center, choose the Active users option

Step 4: First click on the radio button (Try the new admin center) and turn it off, now click on the More option and then Multi Factor Authentication.

Step 5: Now tick on the user and from here you can enable the Multi Factor Authentication as shown in the picture.

By these we can enable Multifactor Authentication