Rename file in DocumentDirectory

The modern way is (url is the file URL of a file in your sandbox):

var rv = URLResourceValues()
rv.name = newname
try? url.setResourceValues(rv)

To rename a file you can use NSFileManager's moveItemAtURL.

Moving the file with moveItemAtURL at the same location but with two different file names is the same operation as "renaming".

Simple example:

Swift 2

do {
    let path = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0]
    let documentDirectory = NSURL(fileURLWithPath: path)
    let originPath = documentDirectory.URLByAppendingPathComponent("currentname.pdf")
    let destinationPath = documentDirectory.URLByAppendingPathComponent("newname.pdf")
    try NSFileManager.defaultManager().moveItemAtURL(originPath, toURL: destinationPath)
} catch let error as NSError {
    print(error)
}

Swift 3

do {
    let path = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
    let documentDirectory = URL(fileURLWithPath: path)
    let originPath = documentDirectory.appendingPathComponent("currentname.pdf")
    let destinationPath = documentDirectory.appendingPathComponent("newname.pdf")
    try FileManager.default.moveItem(at: originPath, to: destinationPath)
} catch {
    print(error)
}