Your athletes are managed from the Manager > Athletes page.
To filter the athlete list to just one squad, select the squad from the dropdown.
For updating of some common information you can edit directly on this page and then press save.
Alternatively you can select an athletes name to edit them in more detail.
On the detail page you can upload an athletes picture.
As well as being able to review and update details such as who this athletes coaches are and what their target body mass is.