You may convert Astral Diamonds to Zen via the Astral Diamond Exchange. To access the Exchange, press Alt on your keyboard to use your mouse then click the third icon from the left on the toolbar at the top of your screen (the icon looks like a Z with a diamond to the side of it).
After clicking the button, the Astral Diamond Exchange will appear:
Here you can either convert Astral Diamonds to Zen or Zen to Astral Diamonds. If you are wanting to convert Astral Diamonds to Zen, you are already at the right screen. Put in the amount of Zen you are looking to buy at the rate of Astral Diamonds you are willing to pay. Look under the Top ZEN Listings to get an idea of what the current exchange rate is. After putting in the unit price (For example, 340 Astral Diamonds each to 100 Zen), the amount of Astral Diamonds (in this case, 34,000 Astral Diamonds) will be taken from your Characters wealth and stored at the Exchange until someone lists Zen for sale at your desired price.
If you make a mistake or otherwise just want to cancel your listing, click the Listings Tab (make sure the Exchange window is open first), find the listing you want to cancel and click the Cancel button.
No matter if you decide to cancel your listing or keep it active until someone agrees to your price, you’ll need to withdraw your Astral Diamonds OR Zen from the Exchange. To do so, open up the Exchange and at the bottom right click the “Withdraw” button. You are now able to withdraw a portion – or all – of your Exchange holdings to whichever character you are currently on (the characters on your account share the Exchange). If you’d rather your other character(s) to have the Astral Diamonds or Zen, then be sure to log on to them first before withdrawing from your Exchange account.