If your whipped cream has gone from luscious and fluffy to yellow and curdled, don't panic. Stop what you are doing to avoid ending up with butter -- if you get to that point, there's no going back. The good news is that you can very easily fix that curdled-looking, over-whipped whipped cream.
Stop whipping. This may seem like a no brainer, but you can’t fix it by whipping it more. As soon as you notice that your cream has become lumpy, turn the machine off or put the beaters down. You can't whip these lumps out; in fact, if you keep going, you’ll only end up making things worse.
Add 1 to 2 tablespoons of cold, unwhipped whipping or heavy cream to the curdled mixture.
Whisk the mixture by hand to incorporate the unwhipped cream until it forms stiff peaks. The whipped cream should now be fluffy, white and ready to use.