Hi Niels,
I don't understand the reason of your question.
If you did a wrong operation, just before, you can undo by clicking on the red arrow, or menu Edit / Undo.
If the blur image becomes from a saved image you can try to Reinforce : menu Filter / Sharpen / Reinforce or the good icon Reinforce
But this good tool can't do miracles in all cases.

Always do a duplicate image to work, and save as an other name to keep an integer original image.