To adjust your main navigational links, head to:
1. Admin 2. Online store → Navigation 3. Click through to the menu you wish to adjust. 4. Select "edit" next to the relevant menu item, or choose the dustbin icon to remove.
Here's an example of where you can see the dustbin icons:
Here's how to edit the text used:
You can drag and drop the menu items too:
Shopify also have these useful guides explaining further:
For more information relating to your theme's header, view this guide.
To disable a top-level link in your navigation bar, you can add a hashtag into the link and the page will then not take your customers anywhere.
Here's how you can do it:
Online store → Navigation
Click through to the menu you wish to adjust
Select "edit" next to the menu item.
Add /# into the link entry field.
This should disable the link:
Or, you could take a look at this tutorial link which shows how to change the collections displayed when the top-level link is clicked on: