Selecting random position in numpy matrix

I have a numpy matrix that is randomly populated with zeros and ones:

grid = np.random.binomial(1, 0.2, size = (3,3))

Now I need to pick a random position inside this matrix and turn it to 2

I tried:

pos = int(np.random.randint(0,len(grid),1))

But then I get an entire row filled with 2s. How do I pick only one random position? Thank you