How do you slide down mobile menu for navbar?

I'm creating a website using Tailwind CSS and GatsbyJS, I finished a navbar and would like my mobile menu to slide down from the top. I have 2 sections for navbar links, one is displayed on mobile and is activated when the navbar button is clicked and sets "isOpen == true". I would like to add some CSS to add "top-0" when isOpen is set to true. Would anyone know how I could do this?enter image description here

enter image description here