Is there a way to downgrade to older Unity version or different API?

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Or to tweak any sort of startup Unity settings.

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Unity apps usually ship all the necessary unity libs/files along with the executables, do you face some sort of problem or? You can sorta tweak unity settings if you somehow get the manifest file for the game archives but usually Unity games are horrible for the purpose of modding unless devs specifically support that

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Just crashing and low framerate.

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there is a UnityCrashHandler64.exe that runs along with the main executable which should write crash logs somewhere nearby, check those logs for a hint, usually it's the drivers and/or security/resource conflicts