If you want to change your WordPress theme, make sure you first install the new theme. Once that is done, here is the quick guide on how to change to the new theme:
First: Backup
Before you do anything, make sure you backup your WordPress website so you can revert to the current version of your website should something go wrong. With that said, here we go:
Change WordPress Theme

First go to Appearance / Themes and pick the inactive WordPress theme you want to change to by hovering over it and then by clicking on Theme Details.

Then, click on Activate.

And that’s it, you should now see your theme listed as Active.
If you then go to your homepage, you should see the new theme used there with the theme’s design. If not, make sure you clear the cache.