string connessione = Request.Form["connessione"].Trim().Replace("''", "'"); 

Configuration config; 
config = WebConfigurationManager.OpenWebConfiguration("~"); 
ConnectionStringsSection c; 
c = config.GetSection("connectionStrings") as ConnectionStringsSection; 
if (c != null) 
{ 
for (int i = 0; i <= c.ConnectionStrings.Count - 1; i++) 
{ 
if (c.ConnectionStrings[i].Name == "ApplicationServices" || c.ConnectionStrings[i].Name == "LocalSqlServer" || c.ConnectionStrings[i].Name == "LocalMySqlServer") 
continue; 

c.ConnectionStrings[i].ConnectionString = connessione; 
config.Save(ConfigurationSaveMode.Minimal); 
ConfigurationManager.RefreshSection("connectionString"); 

}