Thanks for reaching out to us!
Unfortunately, that is a default behavior, you can test is under our live demo site http://demo.theme4press.com/evolve/. Under 992px wide viewport, the main menu will have multiple lines if needed, then it will be changed into a hamburger menu icon under 768px wide viewport.
There is no built-in setting to change that, you might want to customize the theme instead. You can put a custom request here; https://theme4press.com/theme-customization-service/.
Thanks for your patience and understanding.