1. Open the EAC
Click on start and then expand Exchange Server 2016 and click on Exchange Admin Center.
2. Login to the EAC
If you have a certificate warning, click yes to continue and then enter the credentials to login and press enter.
3. Click on Recipients
On the left hand side, click on Recipients.
4. Select a Mailbox
Select a mailbox you want to move and on the bottom right hand corner click on the link "to another database" and complete the migration batch.