If you want to change the specific layer parameters in pretrained neural networks(e.g. ResNet, DenseNet etc), following the procedure will work.
1. Load target pretrained network in workspace
2. Open "Neural network designer (GUI version, newly updated in 2019a)"
3. Import pretrained network model into the neural network designer space (block diagram will display automatically)
4. Change layer properties (eg. input size, filter size etc)
5. Export network model