Is there a way to manually change the design pressure for a stiffener or panel ?
No, there is no way to manually set the design pressure. This is calculated by Hullscant via Section 8 of ISO the Standard and is therefore an output of the program, not an input. All in all, the software should not be considered to be a design tool; it checks conformity to the ISO standard, which in turn is a ‘sizing rule’. i.e. no part of the standard should be used in isolation.