I cant find how to disband military units :(

I don’t know if there is such a function, but I don’t think so. If your military upkeep is to high, you could try attack the capital of a minor with that unit in the hope that it gets killed.

Sorry to bump this, but there is a way of disbanding military units.

Open the military book, then Ctrl+click on the unit you want to disband. You'll get "disbanding..." text next to it, then after you close the book you'll get a confirmation message asking you if you're sure you want to disband. Click Yes and you're done.