[Python] How to change random index positions in numpy array

I have a numpy array that looks like this:

array([[0.5, 0.2, 0.6],
[0.8, 0.1, 0.3],
[0.4, 0.5, 0.4],
[0.3, 0.2, 0.9]])


I want to change 50% of the values in the 3rd column into a random value. How can I do this efficiently? This operation will have to be performed hundreds of thousands of times, so efficiency is very important here. The output can look like this:

array([[0.5, 0.2, 0.2],
[0.8, 0.1, 0.3],
[0.4, 0.5, 0.4],
[0.3, 0.2, 0.1]])


I first thought about isolating this column, then replacing some of the values, and then moving this column back into the original matrix.

last_column = array[:,2]
last_column = change_values_randomly(last_column)
np.c_[array[:,:2], last_column]


How do I change 50% of these values randomly?

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