iOS update the current weight of the user

gistfile1.txt

// Some weight in gram
double weightInGram = 83400.f;

// Create an instance of HKQuantityType and
// HKQuantity to specify the data type and value
// you want to update
NSDate          *now = [NSDate date];
HKQuantityType  *hkQuantityType = [HKQuantityType quantityTypeForIdentifier:HKQuantityTypeIdentifierBodyMass];
HKQuantity      *hkQuantity = [HKQuantity quantityWithUnit:[HKUnit gramUnit] doubleValue:weightInGram];

// Create the concrete sample
HKQuantitySample *weightSample = [HKQuantitySample quantitySampleWithType:hkQuantityType
                                                                 quantity:hkQuantity
                                                                startDate:now
                                                                  endDate:now];

// Update the weight in the health store
[healthStore saveObject:weightSample withCompletion:^(BOOL success, NSError *error) {
    // ..
}];