1. Go to the Update screen
Click the "Update" link in the navigation.
On this screen, only items that were previously imported will be displayed. You can further filter them by Title, Status and Template.
2. Select the items you would like to update
Sitefinity keeps the necessary information for each item, so you just need to choose which items to update.
3. Click the "Update items" button
This will start a process, which is very similar to the import process. Once the update is ready, you will be shown a results page, showing which items updated successfully.