To obtain a legal name change in Missouri, an applicant must submit a petition to the court. The applicant must publish notice of the name change at least once a week for three weeks in a newspaper within 20 days of the court order. The publication requirement may be waived for victims of domestic violence or child abuse. (Mo. Ann. Stat. § 527.290).
For instructions on legal name changes for minors under 18 in Missouri, see NCTE's Name Changes for Minors in Missouri resource.