Go to WeltPixel > Mega Menu Settings > General Settings
Mobile Menu Breakpoint - Insert the breakpoint when navigation menu changes to mobile look and behavior (value in pixels). Ex. 1050.
Note on applicability:
Case 1: You may need the mobile menu to kick in at a higher resolution than default magento 768px. There are mobile devices and small laptops with resolution between 800-1200px where your desktop menu may look crowded on small devices. By setting a higher breakpoint for mobile menu you will be able cover this cases elegantly.
Case 2: Some website designs use mobile menu for both mobile and desktop. You will be able to achieve this easily by setting a breakpoint that will never be reached by your users ex 8000[px], so mobile menu will show permanently.