In XFCE, whenever I needed to customize some menu entry, I would just go to
/usr/share/applications/ and edit the corresponding
.desktop file. This applies the changes globally and whenever the program is updated the modified
.desktop file will be overwritten.
However, there is a “local” approach for this, which will affect only the current user. Copy the
.desktop file you want to edit to
Open it in a text editor, edit and save the changes. Now, the menu will display this local copy.