Struct is the value type, but it's possible to change the property. *It's against a policy.

MutatingMethod.swift

struct Date {
    var year, month, day: Int
    
    init(year: Int, month: Int, day: Int) {
        self.year = year
        self.month = month
        self.day = day
    }
    
    mutating func increment(year year: UInt) {
        self.year += Int(year)
    }
}

var date = Date(year: 2016, month: 1, day: 18)
print("This year is \(date.year).") // This year is 2016.

date.increment(year: 1)
print("Next year is \(date.year).") // Next year is 2017.