Stacked Navigation

#1. All Pages

You can use this code to Custom CSS box

nav.header-nav-list {
    flex-direction: column;
}

Result:

#2. Specific Page

First, you need to find the Page ID.

Next, use CSS code like this

#collection-660587f852bd273e40879262 nav.header-nav-list {
    flex-direction: column;
}