Store releaseDate as a Date instead of a String and then use a DateFormatter to put it in the format you want. You can set a dateDecodingStrategy when you decode your JSON to read in the date in whatever format it comes in and store it as a Date in your data structure.

import Foundation

let jsonData = """
{
    "release_date": "2020-03-10"
}
""".data(using: .utf8)!

struct SampleJSON: Decodable {
    var releaseDate: Date
}

let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
decoder.dateDecodingStrategy = .formatted(formatter)
let decodedJSON = try? decoder.decode(SampleJSON.self, from: jsonData)

//now decodedJSON.releaseData is a Date object instead of a String
//and you can convert it for display into whatever date format you want

let displayFormatter = DateFormatter()
displayFormatter.dateFormat = "EEEE, MMM d, yyyy"
print(displayFormatter.string(from: decodedJSON?.releaseDate ?? Date()))

If you must have releaseDate stored as a String, you can convert it before display with something like this:

import Foundation

let fmt = DateFormatter()
fmt.locale = Locale(identifier: "en_US_POSIX")
fmt.dateFormat = "yyyy-MM-dd"

//first, convert string to Date
let dt = fmt.date(from: "2020-03-10")!

//then convert Date back to String in a different format
fmt.dateFormat = "EEEE, MMM d, yyyy"
print(fmt.string(from: dt))