Hi Salwa,
It is very simple, you can use parameter sweep to change the value of the parameter of a component as you desired and as this parameter is changing the BER also will be changed and you can use the report option in OPTISYSTEM to draw the graph or you can use any engineering program such as MATLAB to draw these data.
In this Example a very detailed explanation of your question, it plots the variation of input power vs BER
Hope this helps!