參考：

iOS形變之CGAffineTransform

iOS 仿射變換CGAffineTransform詳解

如果想要變化順滑一點，可以加個動畫。比如：

- (void)clickSelectBtn:(UIButton *)btn{
    btn.selected = !btn.selected;
    if (btn.selected == YES) {
               
        // 改變箭頭
        [UIView animateWithDuration:0.5 animations:^{
            btn.imageView.transform = CGAffineTransformRotate(btn.imageView.transform, M_PI);
        }];

    } else {
        
        [UIView animateWithDuration:0.5 animations:^{
           btn.imageView.transform = CGAffineTransformRotate(btn.imageView.transform, -M_PI);
        }];

    }
}