There are a couple of things you need to think about here.
First, the closed status often has the "do not allow edit" flag set on it. That means you can't just click "edit" and change things, and you can't use bullk-edit to change things.
Secondly, the resolution field is an odd one which is not on the edit screen and should never be added to it
So, the hard way to do this would be to get all your users to stop using JIRA, enable edit by hacking the workflow, add the resolution field to the screen, use bulk-edit to change the issues, then remove the resolution field from the edit screen and then undo the workflow hack.
A safer option is to add a an "edit resolution" workflow transition from closed to closed (maybe with a condition of "only admins") and make that go through a new "edit resolution" screen, or even just set the resolution in a post-function.
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