If you have any previous version of Spacious running, please quit it before upgrading.
If your browser does not automatically open the file once the download completed, please double click the file icon next to 'Spacious.zip' in your Downloads window. If a warning dialog comes up, click Continue.
If you've got an old version of Spacious installed, you'll be asked whether you want to replace it with the new one. Click 'Replace'.
Eject the disk image, then start Spacious from your Applications folder by doubleclicking its icon. Congratulations! Spacious is now installed and running on your Mac!