Well, your remedy is to file an Amended Return on Form 1040X where the only change will be to request a refund of the amount of the overpayment you wish to have refunded. At this point, if you are required to make ANY estimated payments for 2015, (that are covered by the 2014 overpayment) you'll want to leave the total projected income tax for 2015 as applied to your 2015 taxes and have the balance refunded.
The only issue I would consider before recommending that you file an Amended Return, is the late timing of doing so, give we are already in mid-November. It could take as much as 2+ months for the Amended Return to be processed by the IRS, which brings you into January 2016.
So, one alternative, although not what your first preference is, is to leave 2014 alone and just get your 2015 return filed early, using the e-file process which would get you the balance of your overpaid taxes in a matter of days after you file, and avoid having to file an Amended Return, which would be subject to some degree of review or audit, before the refund is approved.
If your 2014 return includes any sort of tax issue that could raise questions with the IRS, sale of a business, something requiring any degree of estimation (such as the sale of securities where you don't have the actual records of the security's purchase), or a large charitable contribution, or something similar, I would definitely let "sleeping dogs lie" and not file an Amended 2014 return. I'd just get my 2015 return filed as soon as possible after the end of January, 2016, so that your refund will be issued soon after in the normal course of business for the IRS.
Those are the issues I'd consider in making a recommendation as to what you should do.