Go to INVENTORY > PARTS LIST screen and click on the GRN SEARCH option at the top of the screen.
Clicking this will present you with the following screen:
From here you can search for the GRN required using the grid filters at the top of the window. In your case, you can do this by location.
From here you can select the GRN number you wish to update by clicking in the box with the GRN number and then clicking the little box with the dots…
This opens the GRN History record which contains all the details of this specific GRN.
Click on the STOCK RECEIPT EDIT tab to see the current GRN details.