The simplest way to do it is probably as follows:
textBox1.Text += "current value of n is " + (n++) + Environment.NewLine;
This appends the string to new value to the current value of the Text property.
I'm assuming this is a simplified example. If you're actual requirement is more complex, you may want to look at binding the list of values to something like a
and using data templates to format it appropriately. This would be a more normal 'WPF' way of doing things.
You may want to consider looking through a C# for beginners book. This is a fairly basic string manipulation task, and you may benefit from working through the basics.