Assign to the nearest centroid, move centroids to the mean, repeat.
Clusters Start /4, iteration , inertia
K-means alternates two steps: give every point to its nearest centroid, then move each centroid to the mean of its points. Neither step can raise the inertia, so the run always settles, though not always in the best place. Press New start until a run gets stuck above the dashed line; the points are illustrative.