There's a thread for this in The Wizard's Tower. You need to download one of the two tools there to extract the resources from the haven-resource.jar. One is by Boshaw, the other is by TheTrav. After extracting them, you can modify the files you want. When you're done, make sure to rerun the tool to pack it all back together.
If you need more help than this, consult one of the experts there.

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