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Attorneys withdraw, by motion. Pro se people don't withdraw. Are you talking about dismissing a case? When you say "one more thing" like this, this question is not attached to the original question, so this question is totally out of context and difficult to answer.
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I agree with my colleagues. More information is needed. But assuming that what you want to do is withdraw and application or petition that you have already filed, you need to write a letter to the USCIS office handling your case explaining that you want to withdraw the application or petition.