You need copy up-sells.php file from your parent theme, then go to child theme > create woocommerce empty folder and inside create single-product folder, then add in this folder up-sells.php file. After that, you may edit “OUR OFFERS” text in up-sells.php file of your child theme. Don’t forget to activate child theme on your site to be able to see an effect of your changes.
Please read documentation https://codex.wordpress.org/Child_Themes.
Regards