With Swift 5, you can use one of the five following snippets in order to solve your problem.

#1. Using Dictionary mapValues(_:) method

let dictionary = ["foo": 1, "bar": 2, "baz": 5]

let newDictionary = dictionary.mapValues { value in
    return value + 1
}
//let newDictionary = dictionary.mapValues { $0 + 1 } // also works

print(newDictionary) // prints: ["baz": 6, "foo": 2, "bar": 3]

#2. Using Dictionary map method and init(uniqueKeysWithValues:) initializer

let dictionary = ["foo": 1, "bar": 2, "baz": 5]

let tupleArray = dictionary.map { (key: String, value: Int) in
    return (key, value + 1)
}
//let tupleArray = dictionary.map { (1 + 1) } // also works

let newDictionary = Dictionary(uniqueKeysWithValues: tupleArray)

print(newDictionary) // prints: ["baz": 6, "foo": 2, "bar": 3]

#3. Using Dictionary reduce(_:_:) method or reduce(into:_:) method

let dictionary = ["foo": 1, "bar": 2, "baz": 5]

let newDictionary = dictionary.reduce([:]) { (partialResult: [String: Int], tuple: (key: String, value: Int)) in
    var result = partialResult
    result[tuple.key] = tuple.value + 1
    return result
}

print(newDictionary) // prints: ["baz": 6, "foo": 2, "bar": 3]

let dictionary = ["foo": 1, "bar": 2, "baz": 5]

let newDictionary = dictionary.reduce(into: [:]) { (result: inout [String: Int], tuple: (key: String, value: Int)) in
    result[tuple.key] = tuple.value + 1
}

print(newDictionary) // prints: ["baz": 6, "foo": 2, "bar": 3]

#4. Using Dictionary subscript(_:default:) subscript

let dictionary = ["foo": 1, "bar": 2, "baz": 5]

var newDictionary = [String: Int]()
for (key, value) in dictionary {
    newDictionary[key, default: value] += 1
}

print(newDictionary) // prints: ["baz": 6, "foo": 2, "bar": 3]

#5. Using Dictionary subscript(_:) subscript

let dictionary = ["foo": 1, "bar": 2, "baz": 5]

var newDictionary = [String: Int]()
for (key, value) in dictionary {
    newDictionary[key] = value + 1
}

print(newDictionary) // prints: ["baz": 6, "foo": 2, "bar": 3]

You can achieve this by doing the following:

let myDict = ["1": "one","2": "tow","3": "three"]

let mappedKeys = myDict.map { "0" + $0.key } // ["02", "01", "03"]
let mappedValues = myDict.map { $0.value.uppercased() } // ["TOW", "ONE", "THREE"]

var mappedDict = [String: String]()

let zippedArray = Array((zip(mappedKeys, mappedValues)))

for element in zippedArray {
    mappedDict[element.0] = element.1
}

print(mappedDict) // ["03": "THREE", "02": "TOW", "01": "ONE"]

To be more clear, the above code snippet doing the following:

• Mapping the dictionary keys.
• Mapping the dictionary values.
• Create a new empty dictionary mappedDict to append to it.
• Combining mapped keys/values into zippedArray (using zip).
• Filling mappedDict via for-loop.