the NSLayoutConstraint multiplier property is read-only. But if you need to change it, you can replace the constraint with a modified clone like
@implementation NSLayoutConstraint(ChangeMultiplier)
// visal form center http://stackoverflow.com/a/13148012/349514
-(NSLayoutConstraint *)constraintWithMultiplier:(CGFloat)multiplier
{
return [NSLayoutConstraint
constraintWithItem:self.firstItem
attribute:self.firstAttribute
relatedBy:self.relation
toItem:self.secondItem
attribute:self.secondAttribute
multiplier:multiplier
constant:self.constant];
}
@end
and replace it like
NSLayoutConstraint *c = [self.constraintToChange constraintWithMultiplier:0.75];
[self.view removeConstraint:self.constraintToChange];
[self.view addConstraint:self.constraintToChange = c];
[self.view layoutIfNeeded];