By default, the items in the Global Navigation sub-menus are displayed in alphabetical order, like in the screenshot below:

If you would like to change the position of items and apply custom sorting to the sub-menu, this article is for you.

It depends on whether you have Managed Navigation or Structural Navigation applied to your site. Please, check this in SharePoint by going to Site Settings > Navigation.

Option 1

If you have Managed Navigation set up, it will look as in the below screenshot:

In this case, to sort the items, you need to navigate to Site Settings > Term Store Management. From there, you need to:

1. Select the navigation term set you are using.
2. Select the Custom Sort tab.
3. Select the option Use Custom Sort Order.
4. Sort the items manually by selecting their position in the list.
   
   
   

Option 2

If you have Structural Navigation applied, it will look as in the below screenshot:

In this case, to sort the items, you need to scroll the same Navigation Settings page down to find the sorting settings under the Structural Navigation: Sorting and Structural Navigation: Editing and Sorting categories. From there:

1. Choose Sort Manually option in the Structural Navigation: Sorting category:
2. Manually sort the items using the controls available under the Structural Navigation: Editing and Sorting category:

That's it!

You are welcome to check the related article:

How to Create Header Sub-Menus with Drop-Down Lists