Editing Solder mask top(SMT) layer

Hi all,

I am new to the orcad layout tool. I am creating a foot print for a 16QFN IC. A 32 QFN foot print was available to me and I am editing it to make my 16QFN. When I edited the pad size to my specifications, i could see that the SMT layer is also need modification( I assume this is what i have to do now). I need help in how to edit different layers of pcb, especially the solder mask layer.

Thanks in advance,
sreejith.