Java. How do I override initial arguments?

I am in a pickle with a java application. When It launches it uses -xmx 256m and xss 2m. I need to make it use more ram because it keeps crashing when running out of ram. So I use java -jar application.jar -xmx 4G.

Nothing happens, it still uses the built in -xmx 256m

How do I override the initial settings to give this application more ram?

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