Is there a way to mutate a neural network in tensorflow/keras?
Your question is a little bit too vague, but I would assume that coding your own evolutionary algorithm shouldn’t be too difficult for you given what you have done with neural networks so far.
A good starting point for you would be to research the following EA concepts…

Encoding.
Fitness.
Crossover and/or Mutation.
I would like to create a neuroevolution project using python and tensorflow/keras but I couldn't find any good way of mutating the neural network.
I am aware that there are librarys like NEAT, but I wanted to try and code it myself.
Would appreciate it if anyone can tell me something.