You use the UIImageWriteToSavedPhotosAlbum()
function.
//Let's say the image you want to save is in a UIImage called "imageToBeSaved"
UIImageWriteToSavedPhotosAlbum(imageToBeSaved, nil, nil, nil);
Edit:
//ViewController.m
- (IBAction)onClickSavePhoto:(id)sender{
UIImageWriteToSavedPhotosAlbum(imageToBeSaved, nil, nil, nil);
}