code: func upgradeTo(binary string, rel Release) error {
  	expectedReleases := releaseNames(rel.Tag)
  	for _, asset := range rel.Assets {
  		assetName := path.Base(asset.Name)
  		l.Debugln("considering release", assetName)
  
  		for _, expRel := range expectedReleases {
  			if strings.HasPrefix(assetName, expRel) {
  				return upgradeToURL(assetName, binary, asset.URL)
  			}
  		}
  	}
  
  	return ErrNoReleaseDownload
  }
docs: // Upgrade to the given release, saving the previous binary with a ".old" extension.