Just combine them into the same SQL batch, checking to see if the photo has been passed in or is null.

• Do not use dangerous SQL injection. Use proper parameterization instead.
• Don't read the whole file as bytes. Open it as a stream, and pass that into the parameter. Pass DbNull.Value if you don't have a value.
• Use using to dispose the connection and command.
• Do not store the same data in multiple tables. Each table is a single source of truth for a fact (column).
• Do not just swallow exceptions. Catch them at the top-level (ie directly in the UI handler and return an error message to the user.
• Conside using async to make your UI more responsive.

private async void btnupdate_Click(object sender, EventArgs e)
{
    try
    {
        const string query = @"
UPDATE Student
SET RegNo = @RegNo,
    StudName = @StudentName,
    DateAdd = GETDATE(),
    DOB = @DOB,
    Gender = @Gender,
    -- etc
where id = @id;

IF @Photo IS NOT NULL
    UPDATE Photo
    SET StudImage = @Photo
    WHERE id = @id;
";

        using var connection = DBConnectivity.getConnection();
        using var command = new SqlCommand(query, connection);
        command.Parameters.Add("@RegNo", SqlDbType.BigInt).Value = int.Parse(txtRegNo.Text);
        command.Parameters.Add("@StudentName", SqlDbType.NVarChar, 100).Value = txtStudentName.Text;
        command.Parameters.Add("@DOB", SqlDbType.Date).Value = DateOnly.Parse(txtDOB.Text).ToDateTime();
        command.Parameters.Add("@Gender", SqlDbType.Char, 1).Value = cBGender.Text;
        // etc

        using var stream = pho == 1
            ? File.OpenRead(OpenFileDialog1.FileName)
            : null;
        command.Parameters.Add("@Photo", SqlDbType.VarBinary, -1) = stream ?? (object) DBNull.Value;
        await connection.OpenAsync();
        await command.ExecuteNonQueryAsync();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}

It;s not entirely clear if the Photo actually needs to be inserted. If so, do

UPDATE Student
SET RegNo = @RegNo,
    StudName = @StudentName,
    DateAdd = GETDATE(),
    DOB = @DOB,
    Gender = @Gender,
    -- etc
where id = @id;

IF @Photo IS NOT NULL
    INSERT Photo(id, StudImage)
    VALUES (@id, @Photo);