Menus off screen

I’ve just been making my first ever website and using this theme but have tried for hours and hours to figure this out unsuccessfully.

When hovering over the menus the child and grandchild menus go off the right hand edge of the screen. Can you please tell me very explicitly how to fix this?

I am a complete novice with no coding experience although I’m really pleased with the site other than this issue. Please help ??

Thanks