Update a weight after 15 epoch

Dear Sir/Madam,
I am a beginner at the pytorch. I have occurred a problem. I want to update the weight after 15 epoch and keep the same weight till the end of training. How can I do this? Thank you very much.

Show code and tell what you mean

EDIT: Or simply make if statement and after your 15th epoch dont call
optimizer.zero_grad() loss.backward() optimizer.step()