I have modified invoice template but it is not showing quantity unit i.e. box, kg or any.( I have not changed table data only I have modified smaller changes i.e. invoice in center and removed To.).
When you modify the example sales invoice template, you do not get all the functionality of the default template, because the example is not the default template. The default template is not even written in HTML. So yes, it is easy to make little adjustments like you describe, but many features are lost when you do so.
If you edit the sales invoice by switching from one view template to another, you may also notice other changes between them.
As a result, you must either undertake a major coding exercise or accept the default template.