Login to WordPress Admin: Go to your WordPress dashboard (www.yoursite.com/wp-admin).
Go to Themes: Navigate to Appearance > Themes from the sidebar.
Add New Theme: Click Add New at the top of the screen. You can search for free themes or upload a premium theme from sites like ThemeForest.
Upload a Theme: If you’ve purchased a theme, click Upload Theme and select the theme’s .zip file.
Activate Theme: Once uploaded, click Install Now and then Activate to set your new theme live on your website.
For premium themes, explore ThemeForest here.