How to reinstall the theme to its orginal state?

How to reinstall the theme to its orginal state?
I did some change to the hearder template (example: add own logo and name etc….) but somehow when I load the page in mobile mode it gives me a blank white page. (seems like a white layer is covering the whole page)

Therefore, I want to reinstall the theme but when I delete it and re-install it. It is still in the setting that I had change and not the orginal state as first time installion

Can someone point me the correct way to do this? Thanks!