The following tutorial covers changing diazo Flow Theme footer on a Plone 4.2 website.
Overall, Flo9w theme footer can be subdivided into two parts: standard Plone footer(1) and Plone colophon with some theme credits (2).
To edit footer - access Zope Management Interface, and open portal_view_customizations, and plone.footer among long list of available items. Press Customize to enable current footer editing and edit footer text. Do not forget to save changes when you finish.
To hide theme credits (2) and Plone colophon you will need to add a special class to your ploneCustom.css.
#portal-colophon {
display: none;
}
Read our tutorial on how to customize CSS stylesheets in Plone.