Are you sure it is asking you to answer questions you already answered in the past? If has it been some time since you bought something, this is a new requirement, and Apple may be asking you to answer them for the first time. Remember the answers.
If it is the former, then read on:
Method 1 (preferred)
On a computer, go to http://appleid.apple.com and select "Manage your account". Log in with your Apple ID and password, then select Password and Security from the left column.
You need to know the answers to your existing security questions before you can change them or create new questions.
If you do not remember your answers, there may be a link on that page where you can have a reset email send to your "Rescue email address", as long as you provided one in the past. This option will not appear if you have not provided a Rescue email address.
Check all your known email addresses for their response, which will contain further instructions to follow. The email is automated and should arrive immediately. Check your spam / junk folder if you believe you did not receive it.
Method 2 (you cannot perform the above and you're stuck)If there does not seem to be any way to reset your security questions, go here:
Navigate the columns as follows:
All Products and Services > iTunes > iTunes Store > Account Management > iTunes Store account securityAnswer the questions and if required sign in with your Apple ID and password - same one you used to log in on this support forum.
You will find an option to send an email to Apple on that page. Compose a thoughtfully worded and grammatically correct email explaining your dilemma, and politely ask to have your security questions reset. If you are an American I recommend having an English-literate person review your work. There are over 400 million iTunes account holders and Apple receives a great many of these questions every day. Make yours stand out!
You will receive a confirmation email immediately. Check your email spam / junk folder for it because a humanoid should respond in a business day or two. You may need to provide more information before your questions can be reset. These back-and-forth emails may result in a week of correspondence before it is fixed.