The decision whether to file a criminal charge is made by the prosecutor. Private citizens can report suspected crimes to law enforcement, but private citizens do not have the power to file charges. And, no, a private citizen cannot "drop" a charge on line. Your best bet would be to find out who the prosecutor is and speak with him or her and express your wish that the case be dropped.
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You can't do it online or anywhere else. Only the state can drop the charges. You can contact the detective assigned to the case and tell her you want it dropped. But she's almost certainly going to send it to the DA anyway. His best hope is hiring an attorney that regularly handles assault cases. They will work with you to try to get the case dismissed.
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