You can change the title in wp-content/themes/legenda/woocommerce/single-product/up-sells.php
You will find the code: etheme_create_flex_slider($args, __('Our offers', ETHEME_DOMAIN), false, true);
and change “Our offers” title to anything you want.
Yes, it will be overwritten after theme update.
In this case you need create child theme http://codex.wordpress.org/Child_Themes and move the edited file to your child theme directory wp-content/themes/legenda-child/woocommerce/single-product.