Microsoft Exchange Included with Many Mailbox types such as User mailbox, Resource Mailbox, and Shared Mailbox, Sometimes Systems Admin Created a Mailbox then he wants to change type instead of removing and lose the data.
Here the steps you can follow to change Mailbox type:

1- Change Shared Mailbox to User.

2- Change User Mailbox to Shared.

3- User Mailbox to Resource or Equipment.

Modify Linked Mailbox to Normal.

References:
Ref1
Ref2

Hope this helps.