When you install a theme and edit it in Wordpress, this tries to be clever and remember all your customizations so if you reinstall the theme you will have them all there. But sometimes when you fresh install a theme all you want is the original theme (which you have probably hosed with your carefree editing).

These are the steps to reset a theme to its original state.

1. Have a look at the wp-content/themes directory in your wordpress installation. Find the theme you want to reset/fresh install and take note of its name. I will use twentytwentythree in the rest of the instructions here.
2. First of all login as admin, and go to Appearance → Themes, here select and activate some other theme (other than twentytwentythree). Delete the twentytwentythree theme, by clicking on the Theme Details and clicking on Delete in the bottom right of the theme preview.