On your Wordpress Dashboard, access the Themes section by going to "Appearance" > "Themes".
You will then see the active theme on top of the page and the inactive ones right below it.
The WP Niche Raider Theme does not have a Deactivate Button or Link so the easiest way to uninstall it is to first activate any of the inactive themes. Then you should see the WP Niche Raider theme moving to the section where the inactive themes are.
The Delete Link will now appear and you can click it to Uninstall and Remove it from the site.
Once completed, you can go ahead and install the new theme version. You can download the latest version from the Member's Area.