The Rebuild function is found on the same screen as the Data Integrity Check. If an Integrity Check is performed and errors are found, the Rebuild button becomes enabled. Choosing this will make repairs to the database. After this, you would want to re-run the Integrity Check because in some cases, data corrections will require a second round of re-calculations and another Rebuild.

I would strongly suggestion making a backup of the data (very easy to do - File/Backup/Payables Data within Adagio Payables). You can run the DIC and Rebuild on your live data or the backup copy as you wish.