Paksenarrion is right, but I like to tell my stories in pictures
Step 1: On your city over view for both v1.26 and v2.0, click on alliance in the tool bar on the side. Like so:
P.S. I love the color pink <3
Step 2: You should now see various tabs such as "Members", "Properties", etc. Click on "Properties". Like so:
My alliance is just so epic, right? RIGHT? Hitler, Jesus, Chuck Norris, even Cher for goodness sake!
Step 3: You should now be seeing some more tabs such as "Names", " Additional Settings" and "Leave/Delete". Click on "Leave/Delete" like so:
Final step now!
Step 4: Click on "Leave Alliance". Like so:
There you go, Kaumari.
That is how you leave your alliance.
Hope it helped.
-CheezeHead.
P.S. Paint is such a wonderful program :')