There is currently no way to resend messages from a dead letter queue (DLQ) in the Azure Portal, but it is easy to do this from code.
The code above will either run until there are no messages left in the dead letter queue, or until it has processed its maximum number of messages. I use the code above in an Azure Function, which itself is triggered by a Azure Service Bus Queue trigger.
I also strongly recommend Service Bus Explorer for working with Azure Service Bus (https://github.com/paolosalvatori/ServiceBusExplorer).