Installing An Externally Downloaded Theme
An "Externally Downloaded Theme" is literally a theme you have downloaded from somewhere other than WordPress.
For this you will need to have already downloaded the (zipped) theme file.
1. From the Dashboard, on the left-hand side - click where it says "Appearance". You will now be on a page called "Themes".
2. Click at the top where it says "Add New".
3. Then click "Upload Theme" (where it said "Add New" before).
4. Click "Choose File" and look for the (zipped) Theme File on your computer, select it and click "Open".
5. Now click "Install Now".
6. Once it has been installed, just click "Activate".
Now when you visit your site, you will see your new theme is active and the site looks totally different!