The Update model function  enables you to update fitted values in memory and re-use them in Simulator or Optimization.

If no values have been fitted/ modified, the Update icon will be grayed out. After you update a model, the Update icon will be grayed out.   

You can access this from the main menu, on right-mouse-click and with a keyboard shortcut:

In the Simulator menu, select the Save icon. This will reflect your changes in the Excel file or Reaction Lab model.

To save these changes to disk, you should use File>Save in Excel or Reaction Lab.