How to edit and import php file?

Hi there, I’m fairly new to wordpress.

I’ve installed the smartcreen theme and adding a pricing table shortcode. However the currency is in dollars and I would like to change it to £ and so it appears before the numbers.

I’ve been told I need to change the shortcodes.php file but how can I do this? I can’t find it in the wordpress editor, so do I need to change the file from the zip folder? Then import it somehow :S

Thanks for your replies in advance