Systemic Lupus Erythematosus (SLE) is an immune-mediated disease associated with inflammation of multiple organ systems. This study will assess how safe and effective upadacitinib is in treating adult participants with moderately to severely active SLE. Adverse events and change in the disease activity will be assessed. Upadacitinib is an approved drug for rheumatoid arthritis, psoriatic arthritis, and axial spondylarthritis and is being developed for the treatment of SLE. This study is "double-blinded", which means that neither the trial participants nor the study doctors will know who will be given upadacitinib and who will be given placebo (does not contain treatment drug) . This study comprised of 4 sub studies. In Study 1 and Study 2, study doctors put the participants in 1 of the 2 groups, called treatment arms. Each group receives a different treatment. There is a 1 in 2 chance that participants will be assigned to placebo. Eligible participants from Study 1 and Study 2 will enter Study 3 at week 52 to receive specific doses of upadacitinib. Study 4 is a 104-week continued extension if participation is likely to provide a benefit to their SLE. Approximately 500 participants diagnosed with SLE will be enrolled in each of the Study 1 and Study 2 in approximately 320 sites across the world. Participants will receive oral tablets of upadacitinib or matching placebo once daily for 52 weeks in Study 1 and Study 2. Eligible participants from Study 1 and Study 2 will receive oral tablets of upadacitinib once daily for 52 weeks in Study 3. 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, checking for side effects and completing questionnaires.