How to set the Top Menu as default

Hi, I hope I'm not double posting it here, if I do, please redirect me to the correct thread and moderators can just delete this.

I see there are two set of menus in the template. One is the top menu which is located at the top of the screen. And the main menu which is at the left side of the screen.

I want to make the website to have only the Top menu, and disable the main menu. When I did that, the whole website got an 404 error code and the website refused to load correctly. It only works when I set the default menu back to the "Main menu".

How do I solve this?

Thank you

Nevermind, I got it working on my own.
Apparently it's just disabling the modules from the Joomla back-end administration page.

So I'm having a similar issue. I can't seem to get my Top menu to show as the main navigation on top. How did you get this to work?

Have you read our FAQ page?

I have two menu's. the main menu and the kunena menu, a forum.
After installation the kunena menu is default. How to change the main menu back to default (Joomla 2.5)
Yes, i looked at the FAQ but this does not answer my question and I think not the questions from the people above. Instead of saying "have you read our FAQ", a link might be a better solution. Or if you do not know the answer, do not answer it.
If I open my site I see the login form of kunena. I expect, setting the main menu as the default that this form would disappear.

If you set a menu item of the main menu as default, the main menu will be marked as default. There is only one menu item which can be set as default.