Version 1.6 should uninstall via the component manager as you would with any other component.
But if you have manualy removed some parts then you will have to remove everything manualy
Remove everything in red:
Go to: /components/
com_export_content/..
Go to: /administrator/components/
com_export_content/..
And in PhpAdmin remove(drop) export_sections, export_categories and export_content tables.
After that you should be able to install version 1.7.0
