Changing a 3rd party theme

I recently installed a 3rd party theme and am now seeing the following when I navigate my URL;

Fatal error: Cannot redeclare new_index() (previously declared in /home/u407062234/public_html/social/mod/homepage _1_8/start.php:9) in/home/u407062234/public_html/social/mod/sixtysix(1)/start.php on line 29

I need to change back to the default theme since this one isn't working but cannot access the admin space in order to do so - how can I get around this please?