I'm trying to update the cisco ios software on my router 2500 series and the 24 switch? I bought these routers off ebay
Podhillo and JayaKrishna have given good information about the process of updating software. But there is an aspect that they have not mentioned which may be an issue that impacts the original poster. To download software from Cisco web site requires a maintenance contract. Routers and switches bought from Ebay would not have a maintenance contract.