If you want to adjust the width or phase while the simulation is running, then you can use the Slider block. Add the Slider block to the model, link it to the parameters (period, width, delay, etc) of the Pulse Generator block. Start simulation, adjust the slider and you will see the effect on the pulse through a Scope block.
If changes are needed between "scenarios", which may mean from one simulation to another simulation, then the typical way of doing it is to set different parameter values. For example, if you set the period of the Pulse Generator block as "Period", then you set Period=1 in one simulation and Period=2 in another simulation.
