A Study to Evaluate Upadacitinib in Adolescents and Adults With Moderate to Severe Atopic Dermatitis (Measure Up 2)

* Body weight of ≥ 40 kg at Baseline Visit for participants ≥ 12 and \< 18 years of age * Chronic atopic dermatitis (AD) with onset of symptoms at least 3 years prior to Baseline and subject meets Hanifin and Rajka criteria * Active moderate to severe AD defined by Eczema Area and Severity Index (EASI) ≥ 16, validated Investigator's Global Assessment (vIGA) ≥ 3, body surface area (BSA) affected by AD ≥ 10%, and weekly average of daily Worst Pruritus numerical rating scale (NRS) score ≥ 4. * Candidate for systemic therapy or have recently required systemic therapy for AD * Documented history (within 6 months prior to Baseline) of inadequate response to topical corticosteroid (TCS) or topical calcineurin inhibitor (TCI) or documented systemic treatment for AD or for whom topical treatments are otherwise medically inadvisable due to side effects or safety risks

* Prior exposure to any Janus kinase (JAK) inhibitor * Unable or unwilling to discontinue current AD treatments prior to the study * Requirement of prohibited medications during the study * Other active skin diseases or skin infections requiring systemic treatment or would interfere with appropriate assessment of atopic dermatitis lesions * Female subject who is pregnant, breastfeeding, or considering pregnancy during the study