In order to edit or update your organisation's details, first go to the 'My organisations' section of the recruiter dashboard.
To edit the details of any of the organisations you are administrator for (and have permissions to edit for), click on the button that says 'Organisation' with the little image of the pencil on it beside it on the row of the organisation you wish to edit. Once you are on the edit page, you will be able to edit all information you set when you first created the organisation.
To edit either the logo or the header image for the organisation, navigate back to the 'My organisations' view and click on the name of the organisation you wish to edit this information for. You will then be able to edit the images by clicking on them here, replacing the images with new ones.