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It's quite simple. Just go to the issue you wish to delete history for, pick More option and then Delete Secure Clear Field History.
Take under consideration that it will only delete the history, not the current value of the filed. In order to delete everything, delete the current value first and then delete the history.
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Issues → Fields → Custom fields
picking the cog wheel pick by the Secure Field you wish to change , then pressingpick
Configure → Edit Permission scheme → unchecking the box Save field history and clicking Done.
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