Man, Water Elemental unit is a unit, not a hero. It does not have strength. Only heroes have strength.
If you mean damage, go to Object Editor, find the unit, find:
"Combat - Attack 1 - Damage Base",
"Combat - Attack 1 - Damage - Number of Dice"
"Combat - Attack 1 - Damage - Sides per Die"
and set them to whatever you want.
If you don't know what those things mean, you know, units have minimum
damage and maximum damage.
Minimum damage = (damage base) + (number of dice)
Maximum damage = (damage base) + (number of dice)*(sides per die)
If you mean Water Elemental hit points, find "Stats - Hit Points Maximum" in the Object Editor and set it to whatever you want.
If you want to make a water elemental with strength, make a new hero ad give him/her the Water Elemental model.