Hi!
Is there any way I can force the client to update to the latest version via webstart? As far as I understand it he can always download the jnlp-file, edit it and run it offline?
If you update your JNLP, it will automatically update to your most recent version. Or if you just change the JAR file and leave the JNLP it will do the same. Just don’t allow offline play.
But isn’t it possible for the user to download the jnlp-file, modify it to permit offline launch and then run it?
I suppose, but it’s a very very small population of people who know how to do that and that same population could probably do many worse things if they really wanted to. 
What are you trying to accomplish? If it’s about making sure people play the latest and greatest version, I’d say just don’t worry about it. If someone jumps through all those hoops just to play a stale version of your game, that’s not a use case worth supporting.
If it’s about cheating or something like that, I’d integrate something into the game code itself to make sure you’re playing the latest version. You’re not going to be able to impose any sort of security at the JNLP level. And Demonpants is right - most of the people that have the know how and the desire to do that are already looking at and modifying the decompiled source code for your game, figuring out how to pwn you in far more creative ways than you could ever imagine!
I’m not talking about games here. This was a good place to ask though. Sorry to have misled you. 