how to Upload/Download from a HTTPS server through C#

can you suggest me some free https server from where i can check my code. i think there are some restrictions on my side by network dept.

Your code seems to be perfect!
Are you sure that your ID and Password are corrects or even they are correct they have to be valid
Could you try to access from a computer to which you have never previously identified? but change the userID and the password try this, i said that because I have a doubt about the surrounding systems whether they have been configured to disallow certain patterns of HTTP traffic.

Dim fullfiledownload As String
        Dim filefullpath As String

        Dim fileurl As String = "HTTPS url"
        Dim filedownload As String = "filename"
        Dim newfilename As String = "saved filename"
        Dim filepath As String = "D:\"
        Dim userid As String = "userid"
        Dim userpassword As String = "password"

        fullfiledownload = fileurl & filedownload
        filefullpath = filepath & newfilename

        ServicePointManager.CertificatePolicy = New Certpolicy() //returning true for all certificates.
        Dim wc As New WebClient
        Dim myCredentials As New NetworkCredential(userid, userpassword)
        wc.Credentials = myCredentials        
        wc.DownloadFile(fullfiledownload, filefullpath)

this code is giving me error "The remote server returned an error: (401) Unauthorized ". I dont know why.

 I am using Serv-U for making an HTTPS server. and connecting to it.

Sorry to pose another question but I try to understand so I can answer correctly
Is it the unzipped file the endomaged or the zip file it self?

it is a zip file.

what is the nature of the downloaded file I mean the extention

I have added a certificate policy ...but now the application does not give an error but an incorrect file of 31 bytes every time...
On a close look reveals that the repose i get after my download statement is  "Virtual User logged in", and every time i get this error.
I have allowed all certificates in my certificate policy.

Then you miss to implement a Certificate Policy,

When downloading the file from your code. i get the following exception "The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.".
I have given the credentials also, but got no sucess.

WebClient client = new WebClient();
client.UploadFile("yourAdress","PUT","yourfilepath");
oClient.DownloadFile("yourAdress", "yourFilepath");