There is currently no property of the "heatmap" object that allows users to directly change the labels. I work for MathWorks Technical Support and let the appropriate development team know about this limitation so that they will consider allowing more control of labels for future releases of MATLAB.
In the meantime, I suggest you do some preprocessing of the underlying data that you are plotting in the heatmap. For example, you can plot half as much data by skipping every other data point in the underlying table or arrays.
