I want to edit highlited area: for example, change firefox with chromium or remove any item. Menu editor can change only right part of menu, changing default application has no effect on this panel.
All you need to do is click and drag. In the image below, I have selected Firefox and am in the process of moving it down a few places:
To remove an icon just drag it away and to add one navigate to it in the menu and drag it onto the right hand side.