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What you are looking for to edit can be found in System Admin>>Templates>>[TemplateName]>>System>>system_redirect.html
You will need to clone the template before you are allowed to save changes. Also, you will have to set the new template name as your default in System Admin>>Preferences>>General Settings.
You should also be sure that in System Admin>> Preferences>>General Setting that you look for:
"Update module template .html files from themes/your theme/templates directory? Y|N "
Set this to Yes for your changes to be seen right away.
To see it in action on my site
click here.