Change avatar stored in database

Please help with code to replace existing avatar in database. 
I am using vb. Here is my existing code that I use to upload images to database. I need to replace the existing image on button click. If anyone knows any samples that would be great. Thank you.

If FileUpload_ImageUpload.PostedFile Is Nothing OrElse String.IsNullOrEmpty(FileUpload_ImageUpload.PostedFile.FileName) OrElse FileUpload_ImageUpload.PostedFile.InputStream Is Nothing Then Return End If
Dim extension As String = Path.GetExtension(FileUpload_ImageUpload.PostedFile.FileName).ToLower() Dim MIMEType As String = Nothing
Select Case extension Case ".gif" MIMEType = "image/gif" Exit Select Case ".jpg", ".jpeg", ".jpe" MIMEType = "image/jpeg" Exit Select Case ".png" MIMEType = "image/png"
Exit Select
Case Else
'Invali ...