Help me change my code to solve this elliptic pde, pls?

Hi guys, I'm working to develop an algorithm to solve elliptical PDE problems with Neumann and Dirichlet conditions. Attached is my code for problems with Dirichlet conditions, I can't change it correctly to fit a Neumann condition on the right border, $\frac{\partial u}{\partial x}|_{x=xM}=0$. I used the PDE Modeler tool and the result is as shown in the photo. Can you tell me the change I should make or if you already have a code ready, send it to me?