You might mean one of two things.
You want to keep the same Adobe account/ID, but change the email associated with it.
Log in at the Adobe website and go to My Preferences (dropdown under your name at top right)
You shouldn't need to change ADE (Digital Editions).
It should stay associated with the same account, and will (eventually) show the new email when you query the account.
You want to change to a different Adobe account/ID.
Remember if you do this you will no longer have access to books that were authorized under the old account.
You need to deauthorize first, using the secret hidden command ctrl-shift-D (cmd-shift-D on a Mac).
Then authorize with the new account, menu/Help/Authorize Computer (Library/Authorize Computer on older more reliable v1.7.2)
Absolutely major thank you - the 'secret hidden command' ctrl-shift-D worked to de-authorize our old email address and let us add our new one....Thank goodness for the community of very helpful adobe digital editions users - now we can actually put borrowed library books on both our ereaders. No one had answered my query on another thread, but I found this through the 'similar' link.
I have to agree that there is less than helpful information about what should be fairly straightforward tasks. In contrast, the Kobo live chat folks were absolutely patient and helpful when we needed to transfer books from the old email, merge them to the new account and then advised us to go back to 'factory reset' to put in the new email on our first Kobo ereader...piece of cake compared to how long I've been trying to fix our ADE issue.