OleDbCommand command = new OleDbCommand();

command.CommandText = "INSERT INTO tblUsersAccount (Username,[Password],Firstname,     MiddleName, Lastname,Birthday,ContactNo,DateCreated,DateModified) values (@Username,@Password,@Firstname,@MiddleName,@Lastname,@Birthday,@ContactNo,@DateCreated,@DateModified)";

command.Parameters.Add("@Username", OleDbType.Char).Value = tbUsername.Text;
command.Parameters.Add("@Password", OleDbType.Char).Value = tbPassword.Text;
command.Parameters.Add("@Firstname", OleDbType.Char).Value = tbFirstname.Text;
command.Parameters.Add("@MiddleName", OleDbType.Char).Value = tbMiddleName.Text;
command.Parameters.Add("@Lastname", OleDbType.Char).Value = tbLastname.Text;
command.Parameters.Add("@Birthday", OleDbType.Date).Value =DateTime.Parse(tbBirthday.Text);
command.Parameters.Add("@ContactNo", OleDbType.Char).Value = tbContactNo.Text;
command.Parameters.Add("@DateCreated", OleDbType.Date).Value = DateTime.Now;
command.Parameters.Add("@DateModified", OleDbType.Date).Value = DateTime.Now;
command.ExecuteNonQuery();

OleDbCommand command = new OleDbCommand();

command.CommandText = "UPDATE tblUsersAccount SET Password = ?, Firstname = ?, MiddleName = ?, Lastname = ?, Birthday = ?, ContactNo = ? WHERE Username = ?";

connect.ConnectionString = connectionString;
connect.Open();
command.Connection = connect;

command.Parameters.Add("@Username", OleDbType.Char).Value = tbUsername.Text;
command.Parameters.Add("@Password", OleDbType.Char).Value = tbPassword.Text;
command.Parameters.Add("@Firstname", OleDbType.Char).Value = tbFirstname.Text;
command.Parameters.Add("@MiddleName", OleDbType.Char).Value = tbMiddleName.Text;
command.Parameters.Add("@Lastname", OleDbType.Char).Value = tbLastname.Text;
command.Parameters.Add("@Birthday", OleDbType.Date).Value =DateTime.Parse(tbBirthday.Text);
command.Parameters.Add("@ContactNo", OleDbType.Char).Value = tbContactNo.Text;
command.ExecuteNonQuery();

DateTime.Parse(tbBirthday.Text).Date

OleDbCommand command = new OleDbCommand();

command.CommandText = "UPDATE tblUsersAccount SET [Password] = ?, Firstname = ?, " +
                      "MiddleName = ?, Lastname = ?, Birthday = ?, ContactNo = ? " +
                      "WHERE Username = ?";

connect.ConnectionString = connectionString;
connect.Open();
command.Connection = connect;

command.Parameters.Add("@Password", OleDbType.Char).Value = tbPassword.Text;
command.Parameters.Add("@Firstname", OleDbType.Char).Value = tbFirstname.Text;
command.Parameters.Add("@MiddleName", OleDbType.Char).Value = tbMiddleName.Text;
command.Parameters.Add("@Lastname", OleDbType.Char).Value = tbLastname.Text;
command.Parameters.Add("@Birthday", OleDbType.Date).Value =DateTime.Parse(tbBirthday.Text);
command.Parameters.Add("@ContactNo", OleDbType.Char).Value = tbContactNo.Text;
command.Parameters.Add("@Username", OleDbType.Char).Value = tbUsername.Text;
command.ExecuteNonQuery();

command.CommandText = "INSERT INTO tblUsersAccount (Username,[Password],Firstname,     MiddleName, Lastname,Birthday,ContactNo,DateCreated,DateModified) values (@Username,@Password,@Firstname,@MiddleName,@Lastname,@Birthday,@ContactNo,@DateCreated,@DateModified)";

command.Parameters.Add("@Username", OleDbType.Char).Value = tbUsername.Text;
command.Parameters.Add("@Password", OleDbType.Char).Value = tbPassword.Text;
command.Parameters.Add("@Firstname", OleDbType.Char).Value = tbFirstname.Text;
command.Parameters.Add("@MiddleName", OleDbType.Char).Value = tbMiddleName.Text;
command.Parameters.Add("@Lastname", OleDbType.Char).Value = tbLastname.Text;
command.Parameters.Add("@Birthday", OleDbType.Date).Value =DateTime.Parse(tbBirthday.Text).Date;//<---
command.Parameters.Add("@ContactNo", OleDbType.Char).Value = tbContactNo.Text;
command.Parameters.Add("@DateCreated", OleDbType.Date).Value = DateTime.Now.Date; //<---
command.Parameters.Add("@DateModified", OleDbType.Date).Value = DateTime.Now.Date;//<---