If you are using the Outlook desktop application(not the web version), the shared mailbox will automatically appear on the bottom left underneath your inbox.
If you are using O365 OWA / Outlook Online (web version), please follow these steps to access a shared mailbox:
- Login to portal.office.com
- Select the "Outlook" icon after signing in
- Click your initials on the top right of the screen
- Select "Open another mailbox"
- Type in the name of the user who was converted to a shared mailbox, a quick search will pop up as you type, click the users name when it pops up
- Select "Open" and a new window will appear, that will display all of the users new/old emails