At a project level
To change an existing project member's role, simply navigate to 'Members', from the project level. From there you can manage everything to do with your project's team members. To change their role for the selected project, click 'edit' next to their username. You can select a standard role or a custom made role.
At an environment level
To change an existing environment member's role, select the right project then locate the right environment. Click on the ellipsis icon at the end of the row (3 vertical dots). If you have sufficient rights, a menu will appear and you will be able to click on 'Manage Access'. If you do not have sufficient rights and you think it may be a mistake, contact the project owner!
From there you can manage everything to do with your environment's team members and obviously change their role for the selected environment. You can select a standard role or a custom made role.