If a spell states that it may be cast only “after [a player’s] upkeep step,” and the turn has multiple upkeep steps, that spell may be cast any time after the first upkeep step ends.
January 25, 2019 · Ravnica Allegiance
If a spell states that it may be cast only "after [a player's] upkeep step," and the turn has multiple upkeep steps, that spell may be cast any time after the first upkeep step ends.
September 30, 2016 · Kaladesh
If a spell states that it may be cast only “after [a player’s] upkeep step,” and the turn has multiple upkeep steps, that spell may be cast any time after the first upkeep step ends.
February 1, 2010 · Worldwake
Second, the active player gets priority. Players may cast spells and activate abilities.