You are only asked to specify a scenario file the first time you build and run a model in a Simile session. So what do you do if you then want to modify scenario values prior to subsequent runs during the same session?
Simple: select the option Parameters... under the I/O tools menu. 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.