From B2evolution

1. Download a skin from the Skin Directory.
2. Unzip the file. If you're not sure how to do this, try double clicking the file that you downloaded. The end result should be a folder filled with files (e.g., _main.php).
3. Connect to the web space where b2evolution is installed via ftp and upload the folder to the skins folder in your b2evolution install.
4. Go to the Blog Settings tab in the backoffice, choose your blog and change the default skin.