There is no official upgrade beyond Android 4.0.3 for this device. There is a minor firmware update for that version that patches a security hole, but it doesn't upgrade Android.
If you really want to go beyond Android 4.0, AND if you are willing to research things like unlocking the bootloader (which is pretty simple for this tablet) and installing custom ROMs (which is also simple for this tablet, IF YOU'RE CAREFUL), then it's possible to install UNOFFICIAL AND UNSUPPORTED Android 4.1 and 4.4 firmwares for this device. Check out Acer Tablet Forum to find the ROMs, and best of luck.