Changing Themes outside the Dashboard

I was trying a new theme (NewsPress) and once I did I got his error:

Call to undefined function add_theme_support() in …/wp-content/themes/newspress/functions.php on line 9

It’s quite possible my version of WordPress is not equip for that theme. Ok, fine. But now, I get that error when I try opening the site. How can I switch back to old theme if I cannot access my dashboard? Are there files I can change directly to switch between themes?

Please help, as my site is currently down as it stands now.

Thanks,
Geoff