Undecline ALL currently declined WSUS updates....

Undecline-AllDeclineWSUSUpdates.ps1

$wsus = Get-WsusServer
$scope = [Microsoft.UpdateServices.Administration.UpdateScope]::new()
$scope.ApprovedStates = [Microsoft.UpdateServices.Administration.ApprovedStates]::Declined
$updates = $wsus.GetUpdates($scope)
$AllComputersTargetGroup = $wsus.GetComputerTargetGroup([Microsoft.UpdateServices.Administration.ComputerTargetGroupId]::AllComputers)
foreach($computerGroup in $AllComputersTargetGroup.GetChildTargetGroups()){
    foreach($update in $updates){
        $update.Approve([Microsoft.UpdateServices.Administration.UpdateApprovalAction]::NotApproved, $computerGroup)
    }
}