It depends on the iphone 6 case. Some cases are made specifically for the iphone 6s, while others are made to fit a range of different mobile phone models. If you have an iphone 6 case that's not specifically designed for the iphone 6s, it's likely that it will fit, but there may be some slight differences in size or design. For example, the buttons on the side of the phone may be slightly smaller or differently shaped than on the iphone 6s, and the camera lens may protrude from the back of the phone a little more.
If you're not sure whether your case will fit the new phone model, it's always best to check.