Is there a reason you want to go to 3.56? You realize that you will loose all JB/CFW functionality right? I would suggest that you upgrade back to 3.55 OFW, then try 3.56 if you want to. Also, the file needs to be named PS3UPDAT.PUP not UPDATE.PUP.
The problem you are having is that the 3.56 upgrade is an ADDITION to 3.55, not a whole patch. You need to install 3.55 OFW and then add the 3.56 patch. If you can't get this to work through the XMB try it from service mode.