How to write this code without using 11 if-statements?
Why can't you do:

progressLabel.Text = string.Format("{0}%", progressBar1.Value)


As @MNZ pointed out I potentially didn't answer the question exactly how the OP asked.  So here's a slightly modified alternative option:

progressLabel.Text = FormatProgress(progressBar1.Value, progressLabel.Text)

private string FormatProgress(int currentProgress, string progressDisplayed)
{
    if (currentProgress &lt; 0 || currentProgress &gt; 100)
        throw new ArgumentOutOfRangeException("Current progress is not in a valid percentage range of 0 to 100");

    if (currentProgress == 1) 
        return FormatProgress(currentProgress);

    return currentProgress % 10 == 0 ? 
              FormatProgress(currentProgress) : 
              progressDisplayed;
}

private string FormatProgress(int progress)
{
    return string.Format("{0}%", progress);
}


And a unit test to confirm:

[TestMethod]
public void Test()
{
    Assert.AreEqual("1%", FormatProgress(1, "0%"));
    Assert.AreEqual("0%", FormatProgress(0, "5%"));
    Assert.AreEqual("10%", FormatProgress(10, "6%"));
    Assert.AreEqual("80%", FormatProgress(85, "80%"));
    Assert.AreEqual("100%", FormatProgress(100, "90%"));
    Assert.AreNotEqual("75%", FormatProgress(75, "70%"));            
}