How do I unset all of the select boxes for pages

I’m not sure if I understand your problem correctly.

But I think you should click on ‘x’ in every select on this page.

When you do it, this select box will have no selected value any more (it will only contain placeholder saying “Select a page…”).

Just look at attached image. I marked this ‘x’ with an arrow on it!

Click on every ‘x’ button.