In many states, a lawyer is required by the court rules to file a motion to withdraw to formally end the representation of the client, even when the case is over and there is no more work for the lawyer to do. Assuming the case really is over, the motion is a formality and you don't need to do anything, unless there is something the lawyer still needs to do for you on your case.
Even though your case is over, however, the ethical rules require the lawyer to protect your interests. This usually means keeping a copy of the legal papers in your case for several years, in case you need them for some reason, or sending notice to you to explain how you can get a copy of your file from the lawyer.
Every person and every case is unique and the information I provide on this forum is not a substitute for consulting with an attorney licensed in your jurisdiction to obtain appropriate legal advice specific to your situation. Any information I have provided is of a general nature. It is not legal advice and should not be construed as such. I am unable to give you legal advice on this forum. Further, I am not your lawyer and by providing information on this forum, I am not agreeing to enter an attorney-client relationship with you. Communications on this forum are not privileged or confidential and should not be treated as such. It is unwise to disclose information of a confidential or personal nature on this forum. If you are unable to afford an attorney in a criminal matter, the court should be able to appoint counsel for you at no cost to you. Also, most state bar associations can provide contact information for free or low-cost legal advice.
Sometimes the attorney may have a continuing obligation after the case is resolved. For instance if a party loses a civil action the attorney for the prevailing plaintiff may seek to enforce the judgment or may seek to collect the costs and attorneys fees incurred. Your attorney may not have agreed to be responsible for that part of the claim and would therefore withdraw to terminate any continuing obligations.