Hi,
Firstly, you will need to download a suitable software image from CCO. You will need a CCO login with appropriate privileges to do so. Verify that you have sufficient Flash and DRAM for the image you are downloading.
Then, follow the procedure in the following document in order to actually upgrade the code:
http://www.cisco.com/en/US/products/hw/routers/ps233/products_tech_note09186a00800941aa.shtml
Hope that helps - pls rate the post if it does.
Paresh