Try changing the XTickLabels of the axis. To view the current x tick labels, in the Command Window type
get(gca,'XTickLabel')
This should return a list of the labels along the x-axis. Note that gca is for "get current axis" and so we assume that the plot whose x-axis labels that you want to change has focus (not some other figure).
Then to change the labels, just provide a new list of the same size and set it via
set(gca,'XTickLabel',[0 12 24 36 48])
Just remember that your new list must have a replacement for each element in the original list.
