From b2evolution manual

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 "Global Settings" > "Skin install" > "Install new" (this step is needed in b2evo 2.0+)
5. Go to "Blog Settings", choose your blog and change the skin.