I want to remove the side menubar(appering on right side of App) which is provided by Mendix bydefault whem we run any app.
Can we hide above Black Menubar by any means or move it as horrizontal menu at top?
just choose a different layout :D
Yes you have lots of possibilities with this. You can alter them the quickest by changing the lay-out on your page. In the atlas_ui_recources module you find a number of predifined lay-outs that you can use.
For more info check out: https://docs.mendix.com/howto/atlasui/#2-design-elements