Changing existing ones can be a lot harder -- whether it's possible depends on how each individual menu is implemented (there's more than one way to do it!) But yes, the code to do this would need to be in a plugin.
I created a new Submenu but I am not able to edit any existing menus available or provided by Eclipse.
For example if I want to delete the topic of content provided under Help>help Contents>Workbench User Guide. How to edit/modify it?
Is it that I need to edit few things with the existing jar files provided under plugins or there is any definite way to do .