How can make my MainForm refresh?

How can make my MainForm refresh?

I have a MainForm and some components on it (RichTextBox, button, 2
textBoxes).
When I click my button, my application writes some text in the
richtextbox. After making some changes in the richtextBox and clicking
again on the button, I still see the old text on the richtextbox.
How can I make my MainForm refresh and also clear richtextBox each time I
click on the button?
I tried MainForm.Refresh(); and richtextBox.Clear(); but no luck!