"Upkeep +X" is a trait that stacks just like any other "+X" trait, no different than Melee +X or Bloodthirsty +X.
Multiple sources of Upkeep +X stack to form the total sum, which is the amount of mana you must pay during the upkeep phase or destroy the permanent.
Creatures with no upkeep can still gain the Upkeep +X trait, or it adds to the existing Upkeep Trait if there is already one present.