submenu direction

I’m a newbee, so bear with me. I’m trying to set up my website with 2017 and the welcome page shows a picture, filling the entire screen, and the menu at the very bottom of the screen. This is how I want it, but: one menue item opens a submenue, and this submenue doesnt open up above the menue but below, which is below the bottom end of the screen.
Once I scroll down, the menue ends up at the top end of the screen and this behavior makes sense. But without scrolling down the submenue remains hidden and cant’s be used.
Is there a way to configure the submenue such that it opens above the main menue on the start screen and below the main menue once I have scrolled down?
Best,Jochen