You add your level to all of their stats. For example, here is the stat block for the horse companion:
Ability Scores: Strength 14, Constitution 14, Dexterity 16, Intelligence 6, Wisdom 12, Charisma 6
Size: Medium or Large (you choose at creation, but must be one size larger than you if you want to use the Mount ability).
Speed: 10 squares
Defenses: AC 14 + level, Fortitude 13 + level, Reflex 12 + level, Will 12 + level
Hit Points: 14 + 8 per level
Attack Bonus: Level + 4
Melee Basic Attack: Hooves; level + 4 vs. AC; 1d8 + Strength modifier damage.
Trained Skill: Endurance