If you're using PowerShell, you can unmanage an interface using the Unmanage verb, like this (this example will unmanage the interface with ID 11 for 5 minutes, starting now):
return Invoke-SwisVerb $SWIS Orion.NPM.Interfaces Unmanage @( $interfaceID, $start, $end, "false" )
If you want to use the REST API, construct a URL like this (using your Solarwinds server IP to replace localhost):
with a request body like this (again, for an interface with ID 11). Remember to convert the request body to JSON before sending the request. $start and $end are datetimes.
("I:11", $start, $end, "false")