refer below docs:
http://www.cisco.com/c/en/us/support/docs/routers/2500-series-routers/10252-13.html
https://supportforums.cisco.com/document/98421/how-upgrade-or-downgrade-ios-isr-or-similar-router
http://www.cisco.com/c/en/us/td/docs/ios/12_2/configfun/configuration/guide/ffun_c/fcf010.html#wp1001420