Navigate to ancestry.com/secure/account/delete while signed in, and check that the email address shown is yours. Per Ancestry's deletion walkthrough, the page then has you tick three boxes: that the memberships listed will be cancelled, that your trees will be deleted, and that your DNA results will be deleted and the saliva sample destroyed. Choose a reason, click Continue, then Next step; Ancestry emails a verification code that expires in two hours. Keep the tab open, paste the code into the Confirm Account Delete box in it, and click Delete account. Access ends immediately and the data purge completes within 30 days. If you skipped step 2, the DNA data goes in this step too.
An All Access membership's Fold3 and Newspapers.com subscriptions are cancelled with it, but a separate account on Archives.com, Fold3, Forces War Records or Newspapers.com survives and has to be cancelled on that site.