You may find this text in xstore/woocommerce/single-product/up-sells.php file.
Please note you need to make changes in the child theme https://codex.wordpress.org/Child_Themes so the modifications won’t be lost after theme update.
Create the same directory in child theme like xstore-child/woocommerce/single-product and copy the file up-sells.php there and edit it.
Regards