Instead of updating record, you can replace menuitem itself,
All you have to do is find menuitem that you want to override then add your code to it.
For example,
Already defined menu,
<menuitem id="menu_example" action="menu_action" name="Example Menu" parent="menu_example_parent" sequence="10"/>
Now suppose you want to add group to this menu,
<menuitem id="existing_module.menu_example" action="existing_module.menu_action" name="Example Menu" parent="existing_module.menu_example_parent" sequence="10" groups="group_example"/>
If it doesn't work then first delete that menu and after that write that menu again including your code.
For deleting menu,
<delete model="ir.ui.menu" id="module_name.menu_id" />
Hope this is helpful.