Hello! I have almost completed the process for doing this myself. A little backstory, my advisor was randomly assigned to me and has been on sabbatical the entire 3 years I have been in my program. He had only responded to one email in that time frame. I reached out to my program's coordinator and talked with her about the issue. She then offered some suggestion of new advisors based on my research and described the process for changing advisors (for us, there is a form that needs a few signatures). I met with each of the potential advisors and discussed my research interests, my dissertation project, etc. to determine the best fit for me. I am currently to the point of completing the paperwork to change advisors. I'm not sure if the is the "correct' approach, but I hope this is helpful!