You are only asked to specify a scenario file the first time you build and run a model that requires parameter values. So what do you do if you then want to modify scenario values prior to subsequent runs during the same session?
In Simile version 7 and later, the explorer pane in the model run environment window contains a notebook with two tabs, "Outputs" and "Parameters". All the parameter values in the current scenario appear under the Defaults heading in the Parameters tab, and they can be updated directly using any of the methods for setting them.
In earlier versions: select the option Parameters... under the File menu of the model run environment window, or under under the I/O tools menu of the model window if using multiple run windows. This produces the same dialogue window as you use to specify the scenario in the first place. You can edit any of the values you see; or you can re-load another set of data values from a .csv file to replace the ones already entered. You can also load a new scenario file, either for the whole model or for any submodel that contains file parameters.
In either case, if you change any values, you will be requested to restart the model before running it again, so the new values can take effect.