When I answered last time, I didn't have time to write more, thus just giving a hint.
Use the "Modify Protounit" trigger.
There, you can see some menus.
ProtoUnit: Choose the unit.
Player: Choose the player which onws the unit.
Field: Choose what stat you want to edit.
Delta: Write in the amount you want to add to the units respective stat. To remove stats, just write a - before you write the number. This will apply for all the units of the type you chose, as long as the units is owned by the player you chose.
You can't edit the stats of one single unit and let the other units of the type have original stats if they are owned by the same player. If you want to do so, it must be done with AI players, and you make two AI players where one has modified stats and one hasn't.
I think it's what you do.
†
Sweet baby maizes!