Trying to update ipod

I have an iPod Touch, model MC544LL, with version 4.3.5 (8L1) running on it and I'd like to upgrade the OS because most new apps require at least 5.  There is no software update option under the About so I'm not sure what's different about this device and am trying to figure out how and if this device can be upgraded. Looked all over Apple, but no mention of the model or how to upgrade the OS.