For example:

private void SomeForm_Load(object sender, EventArgs e)
{
string cifTime = timeCIF.Text;
string chkCIF = "N";
emailBody.Text = "Example\nCIF = " + chkCIF + "\nTIME = " + timeCIF.Text;
}

private void SomeBtn_Click(object sender, EventArgs e)
{
string[] sLines = emailBody.Lines;
sLines[1] = emailBody.Lines[1].Remove(6, 1).Insert(6, "Y");
emailBody.Lines = sLines;
}

emailBody.Text = emailBody.Lines[1].Remove(3, 2).Insert(3, "Y");