Hi,
You cant reverse / cancel once the physical inventory document is posted. You may create another physical inventory document (MI01) , enter the correct count (MI04) and post the difference (MI07) again. The new physical inventory document may be created for all materials which are counted and enter the count again or for the materials whose count result were wrong.
Once the difference is posted again with correct count, the stock will be adjusted as required.
Regards,
AKPT