You can change the string in file en_US.po (default one), just need to change the string and hit update button.
If you need to change the string to another language pls check the below instructions –
You can learn from http://www.solostream.com/blog/tutorials/translate-wordpress-theme/ about translating your site into your language. Pls make sure you add the entry for your language into wp-config.php file like this –
define ('WPLANG', 'fr_FR');
Pls use the pot files in ‘fitpro/languages’ folder as a template to create .po files.http://www.poedit.net/ -Poedit is the popular free editor for creating .po and the corresponding binary .mo files.
or
You can change in the actual file single-trainer.php.