If you use Windows XP or below you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Picasa program and then press Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Picasa application and click on Uninstall.
After that, install the latest version of the software and download the pictures using the guide provided by the developer. If you have issues editing the photos, check the Edits and effects section from the official web page.
