Right click on the data tip and select 'Edit Text Update Functions...'
Then, callback editor will pop up as shown in figure below:
Edit the highlighted text as shown in figure above as the number of digits you wish to show on data tip.
Then, click on file tab and click 'save as...' to the directory you wish. Close the callback editor window.
Finally, Right click on the data tip and click on 'Select Text Update Functions...' and go to the directory you save your file just now and double click on the .m file you save.