How to edit the hamburger menu’s border radius? 

To modify the corner of the hamburger menu, follow these steps:

Log in to your WordPress admin panel (WP admin).

Navigate to Divi in the dashboard menu >> Theme Builder >> Edit the header >> Edit the row in which the menu module is located >> Design >> Border >> Rounded Corners >> Change the PX value as per your requirement and set to 0 if you want to make the corners square.