As the Org Admin, you can create new user roles in any project your organization is working on.
These instructions show you how to create a user role that only applies for one project. If you want to create a user role that can be used on all projects follow the guide here - Edit a user role for your organization.
Users can be assigned to more than one user role.
You can delete a user role from a project, but before you do, make sure that no users are assigned that role.
Then, on the User Role Configuration page, find the column for the user role you want to delete. Scroll to the bottom of the page. At the end of that column you'll see a dustbin icon. Click it to delete that user role from the project.