Hello VQP,
if you are refering the demo site,you can add the folllowing CSS code inside your child theme’s style.css file or directly inside Additional CSS under Customizer;
#menu-main ul.sub-menu {
text-align: center;
width: 150px;
}
Note:Please adjust width value as per your requirement either by increasing or decreasing the pixel size of the width. If you want to customize the text alignment you can consult https://www.w3schools.com/cssref/pr_text_text-align.asp
Hopefully this works or else please provide the suitable image attachments of the changes you want to make.
Thanks!