How do I fix the DPI scaling on my laptop?

My new laptop has an enormous 1080p resolution and lately I’ve been getting headaches from using my computer, something that’s never happened to me before. So I decided to change the DPI scaling to 125% to make the text larger. Well, I did, but I found I didn’t like it as it messed up certain web pages and blurred and distorted icons, so I decided to switch back to 100%. Now all of a sudden the text on menus and programs (such as the text in the title bar for Firefox or something like that) is really really small, much smaller than the normal 100% size and I don’t know how to get it back to normal. What should I do?