How to update whole firmeware of tplink for w8901g

Sameer, First thing to do is establishing some facts of this router. Please look at the labels at the bottom of the unit and let me know the firmware version. Something like v1, v2 or v4. Pls look here for the example http://www.tp-link.com/lk/Article/?id=46.

You now have to connect the router to a pc with a network cable, that pc is NOT connected to the internet. For instance with a separate laptop, disable the wifi. The network cable connects the router (one of the LAN connections) to the pc.
Invoke a browser and write in the address line 193.168.1.1 with ID admin and password admin. This should open the service program from the router. Go to Firmware upgrade. And follow the rules. Generally you have to check the latest firmware in the TP-Link site and download that file to your pc. IF you have any questions we are always here to help.
Success, Fred

54Mbps Wireless ADSL2 Modem Router Discontinued Products TD W8901G