Depends on your workflow. If your workflow doesn't allow edits once its done, you can't unless you enable it using workflow properties. Can you check the status "Done" in your workflow and see if it has workflow properties added like "jira.issue.editable" to "true". If so, you need to change it to "false".
Again,if you are using default JIRA workflow, you will not be able to edit. But you can copy and edit it.
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