Sometimes you may want to set or clear attributes of an AD object (e.g. the extensionAttributes of an AD user) through Powershell.

To set an attribute:

Set-ADUser -Identity "AnyADUser" -Add @{extensionAttribute15="SomeValue"}

To clear an attribute (i.e. unset the attribute):

Set-ADUser -Identity "AnyADUser" -Clear extensionAttribute15