If you bought the XING Premium membership or the XING ProJobs add-on through the Apple App Store, you’ll have to revoke it directly via Apple too.
Please click on the following link to begin with: https://reportaproblem.apple.com/
Log in with your Apple ID and password if requested to. Then click on "Report a problem" to the right of the XING app logo. Then select "I would like to cancel this purchase" and click on "Cancel purchase".
Apple will reimburse you the amount paid and you will be downgraded to your earlier membership.