Reducing the number of channels

Due to memory issues and since I have simpler data, I would like to reduce the number of channels in the model. is there a dimple way to do so? i tryed to change the numbers in the function 'ConvBlock' but got an error  "RuntimeError: mat1 dim 1 must match mat2 dim 0" . I would love an explanation on how to do this.
Thank you. 