You could rename "approved" by editing the workflow or going to Admin -> Issues -> Status and altering it there. This would affect all workflows that have "approved" in them though.
If you want a new status, just add it to the workflow, move the transitions in and out of the old "approved" to the new status and delete the "approved" from the flow. See https://confluence.atlassian.com/display/JIRA/Configuring+Workflow for details on editing workflows.
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