This is a comment rather than a question - just sharing some experience.
When I tried to update my local server on a Windows machine, I kept getting error “Unable to access jar file”.
Most searches on this topic suggest that the problem is with not starting the command line from the same directory as the server jar file. However in my case the problem was Windows Firewall blocking access. This only became apparent when in desperation I looked for alternatives to starting with the command line.
The alternative way of starting the server on Windows is to associate .jar files with C:\Program Files\Java\jre7\bin\javaw.exe. Then all you need to do is double click on the server file. To use this shortcut, the data folder location must be specified in server properties. If Windows Firewall is interfering it will let you know when you start this way.