Would you like to change the output channels of the last conv layer only?
If so, then you could manipulate the filters (and bias) directly via:
with torch.no_grad():
    model.last_conv.weight = nn.Parameter(model.last_conv.weight[:-1])
    ...

or any other operation besides the slicing.
Manipulating intermediate conv layers would be a bit more tricky, since you would also have to change the in_channels of the successive conv layer.