Write into file from assets folder uwp

I'd like to write text content into a file that is located Assets folder, so I access file but I don't have access to write into it, my code is:

    try {
            //get the file
            StorageFile storageFile = await StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///assets/test.txt"));

            //try to write sring to it
            await FileIO.WriteTextAsync(storageFile, "my string");

            } catch (Exception ex) {
            Debug.WriteLine("error: " + ex);

            }

and I get the error:

    Exception thrown: 'System.UnauthorizedAccessException' in mscorlib.ni.dll
error: System.UnauthorizedAccessException: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
   at MyProject.MainPage.<overWriteHtmlSrcFile>d__6.MoveNext()

Have to mention that I need to change this file due app scenario, or maybe is there a way to create this file in public app folder and then move it in assets.