After working some time with the model and/or the database, it can be a lot of work to make sure they are in sync. E.g. A table has been added/removed, a varchar column has a different max length and so on. The merge functionality in the modeler handle all of this.
Open the model in the modeler and select the DataMap.
Select Tools->Migrate Database Schema.
Enter or select data source information.
All detected changes will be listed as separate tokens.
Select tokens and their direction. "To Model" mean that the change is to be made from the database to the model. "To DB" mean that opposite.
It is also possible to see the generated SQL for the selected "To DB" tokens.
At last, press Migrate to execute the selected operations. Problems will be reported in a dialog. Operations that succeeded will be removed from the list.