This is a study to evaluate pharmacokinetics, safety and tolerability of upadacitinib in pediatric participants with polyarticular course juvenile idiopathic arthritis. This study consists of three parts: Part 1 is multiple-cohort study that consists of two sequential multiple dose groups. Participants benefiting from the study drug with no ongoing adverse events of special interest or serious adverse events will have option to enroll in Part 2. Part 2 is open-label, long term extension study to evaluate safety and tolerability. Part 3 is an additional safety cohort to evaluate long-term safety and tolerability.