Hi there and welcome
If you need to replace the cable then, contact Apple here or take your power adapter to an Apple Authorized Service Provider or Apple Store. They will help you determine which service and support options are available. If you plan to visit an Apple Retail Store, make a reservation for the Genius Bar.
Nearly always these are replaced for you under warranty
If you would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.