2
You can not modify the default template set. You must Clone the default template set first. Then set the new template set as your default template set.
1. administration/system/templates
2. clone default template set and give it a new name
3. go to administration/system/preferences/general settings
4. set the template set you just created/named as the default template set
5. scroll down further and make sure update template/html is set to YES
6. scroll to bottom click GO
7. now go to your mainmenu template html file and modify it if you must and the changes should take affect.
Unless I missed something.
You should probably have a look at the F.A.Q.s and the documentation site as well.
XOOPS F.A.Q.sDOCUMENTATIONQuest