Hi Abdelwahab,
If you wish to assign those weights in the beginning and keep them as constant, you can set the ‘WeightLearnRateFactor’ property as 0 (which defines the learning rate for that layer) for those layers.
For example,
convolution2dLayer(3,1,'Padding',[1 1 1 1],'WeightLearnRateFactor',0);
Or if you want to fix certain weights to some layers in a trained network , then directly assign those layers the values after training the network.
net = alexnet; % or your pre-trained network
layer = net.Layers(1) % here 1 can be replaced with the layer number you wish to change
layer.Weights = randn(11,11,3,96); %the weight matrix which you wish to assign
