How do I put the image on the right side of the text in a UIButton?

Simplest solution:

iOS 10 & up, Swift:

button.transform = CGAffineTransform(scaleX: -1.0, y: 1.0)
button.titleLabel?.transform = CGAffineTransform(scaleX: -1.0, y: 1.0)
button.imageView?.transform = CGAffineTransform(scaleX: -1.0, y: 1.0)

Before iOS 10, Swift/Obj-C:

button.transform = CGAffineTransformMakeScale(-1.0, 1.0);
button.titleLabel.transform = CGAffineTransformMakeScale(-1.0, 1.0);
button.imageView.transform = CGAffineTransformMakeScale(-1.0, 1.0);

iOS 9 & up, Swift: (Recommended)

button.semanticContentAttribute = .forceRightToLeft