These instructions are for Wordpress themes.

Download your chosen theme. You will probably have to unzip it.

Unzip the files to the Wordpress themes folder on your blog’s server.

This will probably be in the htdocs folder, if your server uses Apache, and called something like:
…….\htdocs\wordpress\wp-content\themes

Now go into your admin account for Wordpress.

Pick Presentation, as shown.

Then choose Theme editor, as shown here and you should see the name of the unzipped new theme, with a preview.

Click on it to choose the new theme you want.

At the top of the page you should see a message to say that the new theme has been activated. Click on “view site” to see it in operation.