how to make it fly

is there any trigger that can change a unit from walking to flying? not just the height. thx

You cannot change its classification.
If you add raven form, you can set its height, but it will still register as a ground unit.

False, under unit - add unit classification.

i tried several times to make use of the "unit - add clasification" event
like :

= is stunned
= is able to attack air units
= is a building
= is a summon
- is a gay unit

but nothing happens, it's the same silly unit as before.

False, that wont work, for some unknown lame reason which i duno

False, it only adds clasification, clasification is mainly used by triggers, or by spells. (Ex, triggering unit = flying, invunerable, stunned) (Ex, Must target flying unit)

At which point i remind you that the OP wanted to make a ground unit actually flying.

I'm pretty sure the only way to properly do this would be to change the unit into a flying type: if you're careful chaos can do everything you need. Check out Pitzer-Mike's Ability tutorial.

unit collision off, flying height 200, add classification flying. DONE.

Turning off collision removes it's collision with other obstacles, but it itself is still treated as an obstable.
You cannot truly make a unit flying in all forms if it started not-flying.
Period.

Well you can alwase do the old fashion Trigger swap trick where you just trigger it to replace the unit when you wan't it to fly with the flying unit showing rising off the ground.

That would yield the best results, yes.