You can use FindItemByText to set value of Talerik dropdown:
RadComboBoxItem item = ddlStatus.FindItemByText("complete");
item.Selected = true;
Complete implementation in your case will be something like this:
protected void updatestatus(object sender, System.EventArgs e)
{
if (txtname.Text != String.Empty)
{
if (ddlStatus.Text.Trim() == "Waiting")
{
RadComboBoxItem item = ddlStatus.FindItemByText("complete");
item.Selected = true;
}
}
}