To access the profile section, click on your name in the top right corner of the dashboard.
All your roles and pending invitations are listed under the "Stack roles" section. You can accept invitations either by clicking on the link in the email you've received, or by clicking the tick next to the invitation listed on this page. You can also refuse an invitation by clicking the button with the cross - in this case the invitation will be removed and you will no longer be able to join the stack.
Some of your personal details can be changed by clicking the "Edit profile" button. Username cannot be changed at this stage.
This section also provides you with an ability to change password. This password changed will immediately propagate to other Common Web Platform services.