In addition to changing constants through the sliders you can also change constants using the Change Constants button 
on the toolbar. This will bring up the constant change dialog:
To change a value click on it, then click on the Modify button or press enter. An editing box appears for you to type a value into:
Type in the new value and click on Update or press the enter key. The value will be updated and the model will be simulated with the results displayed.
Click on the Close button when you have finished making changes.
Changing constants in the manner can be helpful when working with highly subscripted models.
