copy past the source code from github and make some changes either in that part:
self.classifier = nn.Sequential(
            nn.Linear(512 * 7 * 7, 4096),
            nn.ReLU(True),
            nn.Dropout(),
            nn.Linear(4096, 4096),
            nn.ReLU(True),
            nn.Dropout(),
            nn.Linear(4096, num_classes),
        )

to modify the classifier or in the make_layer function for the feature extractor part.
But at this point you should just recode the network it won t be very long and way more clear.