Step 1: Login into portal.office365.com
Sign in with your Login ID and Password.
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: Click on the Username a new window will appear on the right side and then click on the Mail.
Step 5: Then, click Convert to Shared Mailbox to convert your regular mailbox to a shared one.