How to Convert user mailbox into Shared mailbox from Exchange admin center
Time to execute: 15 Minutes
Login to the Office365 portal here portal.office.com and click on Admin:
Click on Admin centers then on Exchange:
Now click on Recipients and select desired User and the click on Convert on right side:
Now click on Yes to confirm the conversion:
Login to the Office365 portal here portal.office.com and click on Admin:

Click on Admin centers then on Exchange:

Now click on Recipients and select desired User and the click on Convert on right side:
Now click on Yes to confirm the conversion:
Updated on: 31/01/2023
Thank you!