Changing Values in a List

[DaveG]

I would like to replace all the values (1.3million) in column 3 with 0.00000000. Can someone please advise how I insert this in my code please?

file = 'Z_Before_After.txt'
data = np.genfromtxt(file)
xx = data[:, 0]
yy = data[:, 1]
elevation = data[:, 2]
initial = data[; 2]

with open(file, "w") as output:
    output.write(xx, yy, initial, elevation)

3560181.703 -3384947.321 -5.02245
3560191.703 -3384947.321 -4.88665
3560201.703 -3384947.321 -4.76042
3560211.703 -3384947.321 -4.66599
3560221.703 -3384947.321 -4.60876
3560231.703 -3384947.321 -4.55424
3560241.703 -3384947.321 -4.49973
3560251.703 -3384947.321 -4.35269
3560261.703 -3384947.321 -4.11991