Ulcerative colitis (UC) is a type of inflammatory bowel disease that causes inflammation and bleeding from the lining of the rectum and colon (large intestine). This study will assess how safe and effective Upadacitinib is in treating pediatric participants with UC. Adverse events and change in disease activity will be assessed. Upadacitinib (RINVOQ) is a drug approved in adults for moderate- to severely active UC and is being developed for moderate- to severely active UC in pediatric participants. This study is conducted in 2 periods: Period 1 is comprised of two phases: an 8-week open-label induction phase which means that the study doctor and patients know that participants will receive UPA Dose-A (or the adult equivalent based on body weight) followed by a 44-week double-blind maintenance phase meaning that neither the participants nor the study doctors will know which dose of upadacitinib will be given(UPA Dose B or Dose C). Period 2 is a 260 week open-label extension (OLE) of Period 1. Approximately 110 pediatric participants with moderate to severely active UC will be enrolled at up to 100 sites worldwide. Participants will receive upadacitinib oral tablets once daily or oral solution twice daily at approximately the same time each day, with or without food. Participants will be followed up for 30 days after each phase (i.e. after induction, maintenance, OLE) and only if a participant doesn't continue into the next phase. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.