The steps are pretty simple. Here's a jist of it.
Once you download the 6.2.2 file from the Software Center,
--Copy the PIX Firewall binary image (pixnnn.bin) to the root directory of the TFTP server.
--From the PIX prompt, issue the copy tftp flash command.
--Enter the remote host IP address.
--Enter the PIX binary filename (has the pixnnn.bin name format).
--Type yes
Please refer to the following link for more detailed information:
http://www.cisco.com/warp/public/110/upgrade.shtml#usingcommdupgrade