how can i update this emac

If your iMac meets the hardware requirements, follow the steps mentioned in this article, after you opened it in Safari:

How to get old versions of macOS - Apple Support

Q - " its possible to download net flex "

A - Netflix is a Streaming Service

A - List of Netflix-compatible devices

Hardware Requirement: Intel Core Duo 1.83-gigahertz (GHz) or higher processor; 512MB RAM

Microsoft Silverlight player: Intel-based Macs running OS X 10.4.11 or later. Compatible browsers are Safari 3 (or higher), Firefox 3 (or higher).

HTML5 player: Intel-based Macs running OS X 10.6 or later. Compatible browsers are Safari 8* (or higher), Google Chrome 37 (or higher).

*Note: Using HTML5 player with Safari 8 (or higher) requires certain late Intel Sandy Bridge or any Intel Ivy Bridge or later generation processor Macs running OS X 10.10.