To change a user's role, start by going to the settings from the top right corner, then Account Administration.

In the Users tab, you see a list of all the users in your account. From the roles column, you can change the role of a user, and this will affect the permissions granted to this user.

In the Roles tab, click on a role to open it and see the permissions granted to a user having this role.

If you want to assign a user to this role, go to the Members tab, then click Manage members.

In this dialog, you can add more members to this role. Once done, click Save.

Note that assigning a user to a role will remove them from any other role they were assigned to. In other terms, a user can be assigned only one role.