SEOUL—Samsung Electronics Co. is planning a major technological upgrade for its 10th anniversary flagship phones next year, including next-generation 5G network speeds, bigger screens and more cameras, according to people familiar with the matter.

As with rival Apple Inc.’s 10th anniversary iPhone offerings last year, Samsung’s 10-year reboot of its premium smartphone lineup is seeking to dazzle consumers with more ambition than in years past, these people said, part of a bid by the South Korean technology giant to reverse...