It is difficult to control the acidity of the finished product when you do not have control over the source juice. Cider is all about blending from start to finish. The best you can do is attempt to blend it at this point. There are ways of reducing acidity along the way such as malo-lactic fermentation, but that is a bit tricky and it is too late for this batch anyway.
If you can, the easiest way is to start with a good blend of apples. You want a good mix of neutral sweet, bitter (tannic), and sharp (acidic) apple varieties. If you have too many sharps or bitters you will have a tart acidic or dry tannic cider. Bittersweet and bittersharp varieties are harder to come by these days as hard cider has not been as "fashionable" since before prohibiton in the U.S. and not many producers grow these older varieties as they are really only good for cider (hard) production.
In your case I would recommend fermenting another batch of cider and try and keep it more neutral and not too acidic. Try and get a fairly sweet cider made from neutral medium to low acid apples. Golden delicious is pretty low acid and neutral. It may even turn out pretty bland and insipid by itself, but then you can blend it with your overly acidic cider. Do taste trials using a graduated cylinder and then scale up the ratios to blend the ciders. Then let them sit in a keg or carboy after blending for a while to stabilize before packaging.