there are two ways to join or leave a domain. The netdom command or the Powershell Command lets add-computer and remove-computer.
The netdom way
Join a domain. If you are logged on at the machine you want to join
C:\> netdom join %computername% /domain:your.ADDomainToJoin.net /UserD:LoginWithJoinPermissions /PasswordD:*
Remove from Domain and join a workgroup. The /force option disjoins the computer from the domain even if you to not have the permission to remove the computer object from the domain. The domain computer object remains.
c:\> netdom remove %computername% /domain:your.OldADDomain.net /force
The Powershell way
Join a Domain
PS C:\> Add-computer -DomainName "your.ADDomainToJoin.net" -Credential LoginWithJoinPermissions
Remove from Domain and join a workgroup.
PS C:\> Remove-computer -WorkgroupName yourWorkgroup