Hello. You can only update to 4.0 if the device supports it. Though I had a Galaxy S2 that did.
Let us know what device you have and someone may help more.
If your device is not running properly this may not cure it, but as most apps will have been updated for newer OS versions it may help if available.
A good percentage of Android devices are still running old Android systems. We're only talking 2011 - 2012 here.