Veer,
To download the software you need to have valid CCO id and software contract associated to this router.
Link to download the software is:
https://software.cisco.com/download/release.html?mdfid=282774223&flowid=7438&softwareid=280805680&release=15.1.4M&rellifecycle=ED&relind=AVAILABLE&reltype=all
Upgrade procedure:
Easy way to upgrade the firmware:
1- Download the firmware.
2- Copy the firmware to the router flash.
3- Modify the boot system variable config on the switch to the latest ios.
4- save the config.
5- reload.
Some links:
http://www.thegeekstuff.com/2011/06/upgrade-cisco-ios-image/
http://www.cisco.com/c/en/us/td/docs/routers/access/1900/software/configuration/guide/Software_Configuration/upgrade.html
HTH
Regards
Inayath
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