How do i change the ubuntu main menu or start icon?

What windows manager do you use?

Kde, Gnome, xfce??

Thanks

its GNOME and KDE both so please tell me how to do it in KDE and GNOME.

I only work with GNOME, so this might not work for KDE

Secondly, there are two sides to this:
  - application icons come with an application, the easiest way to change them would be to edit their launcher.
  - icons used by applications come with iconpacks

**** HOW TO CHANGE ICONS OF APPLICATIONS IN THE MENU ****

  1. right-click on the menu .. choose edit-menu
  2. choose an application
  3. right-click, choose properties from the menu
  4. this will open a window with properties about the menu-option
  5. click on the icon in this window
  6. select a different icon

**** HOW TO CHANGE ICONS OF LAUNCERS ****

  1. right-click on a launcher in your panel (like firefox)
  2. choose properties
  3. click on the icon
  4. select a different icon

***** ABOUT ICON PACKS ****

The icons of an iconpack are stored in two different places:
  - icon-packs installed for all users are found /usr/share/icons
  - icon-packs installed for only you are found ~/.icons

You cannot change the files in /usr/share/icons unless you are in root-mode.

**** HOW TO USE A SPECIFIC DOWNLOADED ICON ****

  1. open a terminal (Applications -> Utilities -> Terminal)
  2. type: gksu nautilus
  3. type your password
  4. now a filebrowser window will popup where you are in root-mode
  5. be carefull about what you do to your files now
  6. go to /usr/share/icons
  7. find your icons .. they often have them in several sizes
  8. copy your icon over the original one
  9. make sure they are the same size (each size is in its own directory)

******** GOOD LUCK *********