You need to manually delete the activation file and then resign in. Please follow these steps:
You must have administration authority.
On Windows it can be found here:
Open Windows Registry by using the command prompt and typing 'regedit' or by opening the start menu and searching for 'regedit'.
Search for the following folder:
The 'Activation' folder can be entirely deleted.
On Mac it can be found here:
In Finder, find the 'Go' option on the menu bar and click 'Go to folder' and type the following in:
~/Library/Application Support/Adobe/Digital Editions/
Delete the activation.dat file within the 'Digital Editions' folder.
Restart ADE and it should be deactivated at this point. You should be good to sign back in.