Add dropdown menu to Theme navigation

Hello,

I desperately need the dropdown menu feature in Kula WordPress theme. Anyone here who is willing to help me out? How to get started? I’d like to do it myself. I’d like to learn how to do it.

Theme site: https://themeforest.net/item/kula-responsive-html5-one-page-wordpress-theme/4491523
My site: https://barrebmx.com/

Thanks in advance.