First of all, locate a WordPress theme on WordPress.org or WordPress.com repository i.e.
or simply Google your theme name i.e “Penscratch 2”;
https://wordpress.com/theme/penscratch-2/
Next, click DOWNLOAD button
Next, upload the archived theme file in your own WordPress website and extract it in this directory or folder;
/wp-content/themes/
Finally, go to your dashboard and you should see newly uploaded theme listed in the Appearance –> Theme section;