By default screen-scraper will only allow updates to stable versions (e.g., 2.6 as opposed to 2.6.0.5a). In order to upgrade to unstable versions you need to open the "Settings" dialog box (click on the wrench icon in the button bar), then check the box labeled "Allow upgrading to unstable versions".

After closing the Settings window, go again under Options and choose "Check for updates."

If you're interested in upgrading or downgrading to a specific version (including alpha releases) please see the following instructions.