If you want to change particular word or phrase in the theme (it's when you want to change english to english, not for translation, for translations check THIS solution).
The easiest way is to You can try to use this plugin: https://wordpress.org/plugins/say-what/
Text domain: sepia for Sepia WP Theme, apartment-wp for Apartment WP Theme, biggallery for Big Gallery WP
Another method is to do a translation (from English to English) to change the "transaction". Install Loco Translate plugin and create new English translation. Then find string you want to change and "translate" it to the new one.