Step 1: Sign into your Office admin account via https://portal.office.com
Step 2: Navigate to Exchange admin center.
Step 3: On the left pane, scroll down and click More features.
Step 4: Click open under Permissions.
Step 5: Click Compliance management on the left pane.
Step 6: Switch to the Retention tags tab.
Step 7: Click on the Add icon and select an applying method from the list.
Step 8: Enter name, retention action and period and click Save.