The Rebuild Library Index button should only be applied only to fix certain conditions that include whether an upgrade requires it or Luna's support department recommends it.
When advised to Rebuild Library Index using the Collection Manager, open the Collections tab and select the collection you want to rebuild. Click the Rebuild Library Index at the top of the page.
Next, you will be presented with a dialog box. To continue with the process of rebuilding the Library index click the Refresh button.
The following message will be displayed as the Library index is being rebuilt. Click the Close button to exit the dialog box.