Roles can be (un)assigned either via the "Teams" module or the "Roles" one.
via the "Teams" Module
Via the "Teams" module, select a team by going to the list of all teams, clicking the Actions menu, and choosing "Assign roles" in the Actions column - a popup will be opened where you can search for roles and add them to this team. The same can be achieved by going to the detailed view of the team - tab Roles and clicking on the Plus button to Assign a new role.
Roles can be unassigned by going to the detailed view of the team - tab Roles, clicking on the Action columns of a particular role and selecting the Unassign option,
via the "Roles" Module
To add roles to a team via the Roles module, simply click on the + button at the top right of the menu.
To unassign a role from a team, click on the Actions menu (3 dots) and select unassign.