How to Convert a User Mailbox to a Shared Mailbox

Convert a User Mailbox to a Shared Mailbox

1. Go to the Office 365 Admin Center.
2. On the Active users page, select the user whose mailbox you want to convert.
3. On the user's page, expand Mail Settings and click Convert to shared mailbox. You won't see this option if the user doesn't have an Office 365 license assigned to them. Go assign them a license then come back to this step. 
4. Click Convert to complete the conversion.

Reference Link: https://support.office.com/en-us/article/convert-a-user-mailbox-to-a-shared-mailbox-2e122487-e1f5-4f26-ba41-5689249d93ba