There are a few ways to install themes.
1. Easy Way
-go to your WP-Admin control panel of your blog.
-On the main dashboard, click “change theme”
-Click the tab “install themes”
-Search the theme name you want, and click “install”.
-Then click the “Manage Themes” tab at the top.
-Activate the theme you want.
2. Easyish Way
-Dashboard of WP-Admin
-Change Themes
-Install Themes Tab
-Upload Tab
-Upload the theme file (.ZIP)
-Manage Themes Tab
-Activate It
3. If you can’t find the theme in the database of wordpress, follow these instructions.
-Keep the .ZIP file of the theme you downloaded.
-Go to the file manager of your wordpress blog site (Or use FTP)
-Go to “wp-content >> themes”
-Upload the .ZIP file to there.
-Extract the .ZIP so it creates a folder of the theme name. You will know you do this correctly if you see the theme name as a folder along with the rest of the default theme folders (twentyten, etc.)
-Once you have the theme folder under “themes” go to your WP-Admin control Panel again.
-Click “Change Themes”
-WordPress will automatically find the theme folder you just added if you did everything correctly.
-Click “Activate” on the theme you want!
Hope this helped!