Once an order is processed in Linnworks and subsequently moved to the Processed Orders screen, the order cannot be "unprocessed" and no details can be edited on the processed order.
The system expects that processing an order means that all the needed actions on the order have already been done. The only available options for a processed order are creating RMAs, Refunds, re-printing labels and invoices and sending manual emails to the customer email address on the order.
If the order was accidentally processed without the invoice or shipping label having been printed first, then this can be easily amended by using the Reprint Invoice or Reprint Label options. However, note that this option will only work if all of the order, item and shipping details are already correct on the order.
Even if no consignment had been previously created for the label, then using Reprint Label will create it so using this option will allow you to print the missing shipping label for the accidentally processed order.
If e.g. the Shipping Service or order weight is not what you would have needed them to be when printing the shipping label, then you would not be able to edit them before using Reprint Label.
In this case, you would either need to create a Direct order with the same details or e.g. a Resend of the same order. Full details on creating Resends are available here.