Edit completed task

How can I edit a completed task?

I added the task to my invoice, then saved the invoice. I then realized I wanted to remove the VAT from the task, but I can’t edit the task anymore.

I tried to remove the task from the invoice, and save the invoice again. But the task remains “Invoiced” although it’s not attached to any invoice anymore. So How can I edit this task and re-add it to my invoice?