A Study to Evaluate the Pharmacokinetics, Safety and Tolerability of Upadacitinib in Pediatric Participants With Severe Atopic Dermatitis

* Participants with total body weight of 10 kilograms(kg) or higher at Baseline. Beginning with protocol version 6.0, only subjects 3 years of age and older will be enrolled for the remainder of this study. * Diagnosed with atopic dermatitis (AD) with onset of symptoms at least 6 months prior to baseline. * Meets Hanifin and Rajka criteria for AD. * Diagnosed with active severe AD defined by Eczema Area Severity Index (EASI), Validated Investigator's Global Assessment (IGA) and body surface area (BSA). * Documented history (within 12 months prior to the Baseline Visit) of inadequate response or intolerance to topical corticosteroids (TCS) and topical calcineurin inhibitor (TCI) OR for whom use of TCS and TCIs is otherwise medically inadvisable.

* Prior exposure to Janus Kinase (JAK) inhibitor. * Requirement of prohibited medications during the study. * Current use of known moderate or strong inhibitors or inducers of drug metabolizing enzymes within 30 days prior to the first dose of study drug and through the end of Part 1 of the study.