Moving this discussion to the Adobe Digital Editions forum.
You can deactivate digital editions by using Ctrl (CMD) - Shift - D, You can deactivate attached devices by using Ctrl (CMD) - Shift - E on Mac.
For devices where you get a client through the device Market Place (ie iOS, Android), you will need to refer to the application instructions for doing a deactivate.
However deactivating does not get you any more activations, it's only useful if you want to get rid of the device, outside of that, not so much...
You can either wait about 1/2 year to get a new device allowance automatically, or if you can ask Adobe for an Activation reset, if you are getting the E_ACT_TOO_MANY_ACTIVATIONS error