in_plugin_update_message.php

function prefix_plugin_update_message( $data, $response ) {
	if( isset( $data['upgrade_notice'] ) ) {
		printf(
			'<div class="update-message">%s</div>',
			wpautop( $data['upgrade_notice'] )
		);
	}
}
add_action( 'in_plugin_update_message-your-plugin/your-plugin.php', 'prefix_plugin_update_message', 10, 2 );