The easiest way, considering how it is implemented in this tutorial, would be to create an instance variable (text) for the Grid object called "position" and give it a value equal to the correct letter/piece for that position. So, first Grid on top row would be A, the second B, the third C, etc. Last grid is empty, so its position variable would be blank.
Then, every time you reposition a piece (on the click event) you would call fuction similar to this one:
EDIT: Here's a capx, so it's more clear.
https://www.dropbox.com/s/r8rnbl1128oa9 ... .capx?dl=0
I didn't implement the whole game, just the victory condition. Just drag all letters to the grid objects in the correct order. If you want to test, drop one letter on the last grid, which should be empty, put all others on their correct positions and then drag the last one to the right place.