Here's one way to get there using code:
EasingDoubleKeyFrame keyFrame = ((this.Resources["myStoryboard"]
as Storyboard).Children[0]
as DoubleAnimationUsingKeyFrames).KeyFrames[0]
as EasingDoubleKeyFrame;
keyFrame.Value = 100.5;
That said, you should be binding it to a property in the view-model and then changing that.