//labels: lbl_01_temp, lbl_02_temp, lbl_03_temp
string XX;
double id_01_temp, id_02_temp, id_03_temp;
lbl_XX_temp.Text= "The Device " +XX+ "has" +id_XX_temp+" ℃";

lbl_XX_temp.Text= String.Format("The Device {0} has {1} ℃", XX, id_XX_temp);

string lblSelected = String.Format("lbl_{0}_temp", XX);
Label lbl = (Label)this.FindControl(lblSelected);
lbl.Text = String.Format("The Device {0} has {1} ℃", XX, id_XX_temp);

string lblSelected = String.Format("lbl_{0}_temp", XX);
Control[] ctrl = this.Controls.Find(lblSelected, true);
Label lbl = ctrl[0] as Label;
lbl.Text = String.Format("The Device {0} has {1} ℃", XX, id_XX_temp);

labels[index].Text = "Device " + (index + 1) + " has temperature "
    + temperatures[index].ToString(formatString) + " ℃";

for (int i=1; i<=6; i++)
{
    Label label = GetLabelForIndex(i);
    //do something with the label here
}