How did justinian gain the throne?

his father passed on the throne to him.
his uncle made him co-emperor.
his army took control of constantinople.
his wife was of royal birth.

Justinian ruled as co-emperor with Justin I, his uncle. In Justin's later years, when his health began to fade, he named Justinian as his successor, so Justinian thus inherited the entire Byzantine Empire, as Justinian did not appoint another co-emperor after his uncle's death.