developer/api/v1/versions/Unpublish-Version.ps1

function Unpublish-Version {
    Param(
        [Parameter(Mandatory)]
        [string] $AppAlias,
        [Parameter(Mandatory)]
        [string] $VersionAlias,
        [Parameter(Mandatory)]
        [ValidateSet("Definition")]
        [string] $Stage
    )

    return Invoke-Api Post "/developer/api/v1/versions/$AppAlias/$VersionAlias/?action=demote&stage=$Stage"
}