import-module msonline
(install-module msonline)

$O365CREDS = Get-Credential
$ONPREMCREDS = Get-Credential #used UPN Username

$SESSION = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell -Credential $O365CREDS -Authentication Basic -AllowRedirection

Import-PSSession $SESSION
Connect-MsolService -Credential $O365CREDS

#You can run these commands, to check for old MoveRequest or all mailbox in Office365, just remove the hash.
#get-moverequest
#get-mailbox

get-mailbox -id MAILBOX | New-MoveRequest -OutBound -RemoteTargetDatabase DB01 -RemoteHostName owa.domain.prefix -RemoteCredential $ONPREMCREDS -TargetDeliveryDomain ‘InternalAdDomain.prefix’
#Replace MAILBOX with the mailbox
# Replace owa.domain.prefix with the public owa address
# Replace InternalAdDomain.prefix with the internal ad domain name

#You can then run the following commands to check the mailbox moverequest status
Get-MoveRequest | Get-MoveRequestStatistics

Be aware that the user need to be an locally AD users, before you can migrated it back. It can’t be a Cloud users.
See here how to connect the user to an AD user: https://www.codetwo.com/admins-blog/how-to-merge-an-office-365-account-with-an-on-premises-ad-account-after-hybrid-configuration/