Finding a random element in an 2D array,then performing operations on it

I need to find a random element in a 2D array, as the title says.Then i need to perform an operation on this.

For example:

Grid = np.zeros((64,64))
ones = np.ones((32,64))
minus = -1 * ones
Grid[0:32,:] = ones[:]
Grid[32:64,:] = minus[:]

So its half full of 1's,half of -1's.Then i need select a random element of this 2D array,and perform an operation on it. For example it needs to do something like:

def change(Grid):
    Grid[0,0] *= -1

But for a random element inside this list,but obviously the below code doesn't work.

Grid[random.choice,random.choice] *= -1

Is there a way to do this with random.choice,or is there another way to make this work?