//Update an account with the UpdateAccount method
var client = new BFSServiceReference.bfsapiSoapClient();
var credentials = new BFSServiceReference.Credentials()
{
    UserName = bfsusername, //Username of administrative user in your instance of BFS
    Password = bfspassword, //Password of the administrative user in your instance of BFS
};
var response = client.UpdateAccounts(new BFSServiceReference.UpdateAccountsRequest()
{
    Credentials = credentials,
    identify = bfsidentifier, //Identifier is a unique token for your instance of BFS
    Fields = new UpdateAccountFields()
    {
      OwnerAccountLabel  = true,      
    },
    Entities = new []
    {
      new UpdateAccount()
      {
          BrickId = bnuseraccountid,
          OwnerAccountLabel = "Test2",         
      },
    },
    
});
foreach (var c in response.Entities)
{
    Console.WriteLine(c.BrickId + "," + c.OwnerAccountLabel + "," + response.Message);
}