<install path>\exporters\html5\export.html
I wouldn't recommend changing it though - just make your changes after exporting a project, don't modify the source in the install directory, because:
- if you make a mistake it affects everything you export
- when you install a new build it'll be overwritten anyway
- if you change something and it causes a bug, then you report a bug and we can't fix it, much confusion will ensue.