Yes, you change the variables by clicking on the "Edit" button and then enter an expression to change it.
E.g. to change HSCPV[1] to value 0, just write in:
HSCPV[1] = 0
and then press <Enter>. The value of HSCPV will then be changed to the new value. However, note the action where HSCPV passing through a set value to trigger a custom function can not be simulated because that is an interrupt-driven phenomenon. You can test the result of a custom function by connecting an unused input or internal relay bit to the custom function and trigger it by left clicking on that input. That will allow you to simulate the action.