I have Azure subscription and It has 3(guest) owners, I want to change the role of one owner to contributor. I can't find any way to do it. can someone please help me in this.
As such you cannot change the role assignment. What you have to do is delete the existing role assignment (Owner in your case) for the user and create a new role assignment (Contributor in your case).
To do so, go to Azure Portal and then go to the Subscription where you want to make this change. Click on "Access control (IAM)" and then "Role assignments" tab. You will be able to make the changes there.