Rebuilding either the model base, or the historic base, are features that allow you to incorporate larger changes you have made to foundational aspects of IFs. Rebuilding the Model Base (also known as the Futures Base) will ask IFs to reconstruct all of its data from 2000-2100. Rebuilding the Historic Base will ask IFs to reconstruct its data from 1960-2000.
To access Rebuild Model Base, click on Extended Features on the Main Menu of IFs. There you will find two sub-sub-options: