How to disband military units?

I find that I have too many units, so I would like to deactivate some of them and transfer their weapons to other units. They're all mixed up with all kinds of different weapons, and I can't make heads or tails of them, so I want to consolidate them into fewer but larger units, until I understand what each weapon does.

(Please only tell me how to deactivate units, and don't try to persuade me that I shouldn't do that. I suspect that there's an advantage in mixing different weapons into one unit, but I don't care at this time. I will mix them up later, when I know how they work. Right now, I only want to bring some order into what, to my limited brain capacity, seems like utter chaos. Thank you!)

There is a "Dismantle" button above where you view your units. Toggle that, and you can dismantle single units or multiple, similar to how you would manage your email.

Thank you!
What happens to the equipment of dismantled units? Is it transferred to some pool, distributed to other units, or does it vanish?

The weapons, ammo, fuel, etc. are returned to your country stock, and immediately available for different use. Your country can store up to 20,000 weapons of different types for each of the governing military base/airport of the appropriate type. If you exceed that capacity, weapons will start to go "inactive", a sort of lower-maintenance reserve, but those weapons cannot be used until "reactivated".

I see that all units are a mix of at least two different kinds of weapons. Is it possible, or even advisable, to create units of purely one kind of weapon?
Considering that I only intend to attack computer-controlled countries, that is.

Some restrictions that aren't apparent until you form units often prevent that. Some ideal units are ones that are formed of the weapon doing the shooting and another weapon that will absorb your losses. The weapon type taking losses is often "tougher" in that it is hit less often/takes less damage against the weapons you are up against.

For these reasons, "pure" units of one weapon type are rare and often not even optimal.

Edit: 2 examples of common units that do work are:

Fighters/Precision Bombers to clear enemy interceptors. Fighters shoot and bombers absorb your losses.

Offensive AA Batteries/Mid Rand Missile Batteries to clear enemy helicopters. Similarly, AA batteries shoot and MRMBs take your losses.

Aries, I checked it and you are right.
It was not intended to be so but I do understand why it happens.

Let it be.