Participants, 6 to <12 years of age, will receive low dose of upadacitinib.

Participants, 6 to <12 years of age, will receive high dose of upadacitinib.

Participants, 2 to <6 years of age, will receive low dose of upadacitinib.

Participants, 2 to <6 years of age, will receive high dose of upadacitinib.

Eligible participants who completed Part 1 will receive weight-dependant low dose of upadacitinib.

Upadacitinib will be administered orally.

It is defined as the maximum observed plasma concentration (Cmax) for upadacitinib.

It is defined as the time to maximum plasma concentration (Tmax) of upadacitinib.

The area under the plasma concentration-time curve (AUCtau) is a method of measurement of the total exposure of a drug in plasma.

Clearance is defined the volume of plasma cleared of the drug per unit time.

An adverse event (AE) is defined as any untoward medical occurrence in a patient or clinical investigation participant administered a pharmaceutical product which does not necessarily have a causal relationship with this treatment. The investigator assesses the relationship of each event to the use of study drug as either probably related, possibly related, probably not related or not related. A serious adverse event (SAE) is an event that results in death, is life-threatening, requires or prolongs hospitalization, results in a congenital anomaly, persistent or significant disability/incapacity or is an important medical event that, based on medical judgment, may jeopardize the participant and may require medical or surgical intervention to prevent any of the outcomes listed above. Treatment-emergent adverse events (TEAEs) are defined as any event that began or worsened in severity after the first dose of study drug.