Add string before Date in meta data.

functions.php

/**
 * Add prefix to your Date String.
 *
 * @param string $output Markup date.
 * 
 * @return void
 */
function your_prefix_post_date( $output ) {
	
	$output        = '';
	$format        = apply_filters( 'astra_post_date_format', '' );
	$time_string   = esc_html( get_the_date( $format ) );
	$modified_date = esc_html( get_the_modified_date( $format ) );
	$posted_on     = sprintf(
		esc_html( '%s' ),
		$time_string
	);
	$modified_on   = sprintf(
		esc_html( '%s' ),
		$modified_date
	);
	$output       .= '<span class="posted-on">';
	$output       .= '<span class="published" itemprop="datePublished"> ' . $posted_on . '</span>';
	$output       .= '<span class="updated" itemprop="dateModified"> ' . $modified_on . '</span>';
	$output       .= '</span>';
	return 'Update on ' . $output;
}

add_filter( 'astra_post_date', 'your_prefix_post_date' );