Setting amount of experience required
Behavior - Experience Fraction determines what gives XP to the hero (example: 1 kill == +1 XP).
Behavior - Veterancy Levels + is used to set the levels' properties. From there, add a value. Note: each new value is a new level.
You can set the amount of experience required for each level via Minimum Experience. Note: the value of this field represents the experience required to level up. If you want your unit to level at 25 kills, 50, 75, etc. (and have 1 kill == +1 XP), you'll set the Minimum Experience values of all your levels to "25"(and not 25, then 50, then 75...).
Note: Be aware that since you might want to create a lot of levels for your hero, you can copy-paste the level values.
Adding stats boosts (Attributes) to the levels
In order to give +10 damages and +50 HP to your hero every time he levels, you'll have to create an "Attribute" type Behavior.
Once you've created it, go to "Behavior - Modification +", edit the "Unit -> Vital Max Bonus -> Life" to 50 and the "Combat -> Damage Dealt (Unscaled) -> Melee + Ranged" to 10.
Once that's done, go back to your Veterancy Behavior and for all level values, add to the "Modification -> Behavior -> Attribute Changes" your Attribute Behavior. Set its "Points" to 1.
If at some point you want the new levels to make the stats evolve two times faster, set this "Points" to 2.
Setting the hero unit
Obviously, you still haven't given this Veterancy Behavior to your hero unit.
Go to Units tab, find your hero unit, go to the "Behavior - Behaviors +" field and add all the behaviors that are linked to the Veterancy one, inclusively. This means that in our case, we'll add the Veterancy Behavior and the Attribute Behavior.