Center Constraints (AutoLayout iOS)
Select your button (or whatever view you want to center) on the storyboard. Then click the align button on the bottom right. Select Horizontally in Container and Vertically in Container. Click “Add 2 Constraints”.
If it wasn’t perfectly centered already, you may need to do one more thing. Click the “Update Frames” button that is two to the left of the “Embed In Stack” button on the bottom bar.