While the simulation is running, if you double click the Constant block, change its value from one numeric value to another (e.g. from 1 to 2) and hit return, it will take effect immediately.
For a variable constant, there is one trick. Double click the Constant block so it shows the varialbe name. The numeric value is also displayed on the right. Go to MATLAB Command Window, change the value of the variable. Then at the Constant block dialog, you need to do some trivial change such as adding a whitespace at the end of the variable name and then backspace. You will see the displayed numeric value change. Then you hit return and the change will take effect.
Try this on a simple model first.
