In 2018, director Pete Docter mentioned in an interview that there had been discussions about a sequel but nothing was concrete at that time. He said that he personally feels like the story of Up is complete, but he wouldn't rule out the possibility of revisiting the characters in some form in the future.
In 2019, there were reports that Disney and Pixar were considering a prequel to Up that would focus on Carl Fredricksen's early adventures before the events of the first movie. However, this project has not been officially confirmed either.
Until there is an official announcement from Disney or Pixar, the existence of a sequel to Up remains uncertain.