Well, since the text ‘Download Resume’ is hard-coded inside a theme file you have to override this behavior by making a Child Theme.

After making a Child Theme and activating it you would have two new files namely style.css and functions.php.

In your functions.php put this custom code just below your existing code.

    function biography_after_text_slider() {
        global $biography_customizer_all_values;
        $biography_header_contact_url = $biography_customizer_all_values['biography-header-contact-url'];
        $biography_header_resume_url = $biography_customizer_all_values['biography-header-resume-url'];
        if( empty( $biography_header_contact_url) && empty ( $biography_header_resume_url ) ){
            return;
        }
        ?>
        <div class="goest-btn">
            <?php
            if( !empty( $biography_header_contact_url) ){
                ?>
                <a class="button button-feature button-contact" href="<?php echo esc_url( $biography_header_contact_url ); ?>">
                    <?php _e( 'Contact Me', 'biography' );?>
                </a>
                <?php
            }
            if( !empty( $biography_header_resume_url) ){
                ?>
                <a class="button button-feature button-resume line-btn" href="<?php echo esc_url( $biography_header_resume_url ); ?>">
                    <?php _e( 'Buy My Book ', 'biography' ); // changing to your custom text?>
                </a>
                <?php
            }
            ?>
        </div>
        <?php
    }

Please notice in the code above, I have changed the default text to your custom text for button. This should work for you.

Thanks