As with all date manipulation you have to use NSDateComponents and NSCalendar

NSDate *now = [NSDate date];
NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier: NSCalendarIdentifierGregorian];
NSDateComponents *components = [calendar components:NSCalendarUnitYear|NSCalendarUnitMonth|NSCalendarUnitDay fromDate:now];
[components setHour:10];
NSDate *today10am = [calendar dateFromComponents:components];

in iOS8 Apple introduced a convenience method that saves a few lines of code:

NSDate *d = [calendar dateBySettingHour:10 minute:0 second:0 ofDate:[NSDate date] options:0];

Swift:

let calendar: NSCalendar! = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)
let now: NSDate! = NSDate()

let date10h = calendar.dateBySettingHour(10, minute: 0, second: 0, ofDate: now, options: NSCalendarOptions.MatchFirst)!