Step 1: Sign into Office 365 admin portal via https://portal.office.com
Step 2: Click on Admin from the left pane and navigate to Exchange admin center.
Step 3: Click on the Recipients and switch to Shared tab.
Step 4: Click on the shared mailbox you would like to convert.
Step 5: On the right pane, under the Convert to Regular Mailbox section, click on Convert.
Step 6: Click Yes.