I just want to ask on how to change a word to another word and change its color in a textbox. For example if I type the word "Yes" in the text box it will change to "NO" and change its color into red at the same time.
Changing the color is a different matter. I assume you want to change only the replaced text to red, which won't work unless you're using a rich text box. If it's just a regular text box then you're limited to a global foreground color change. But that can be done by changing the ForeColor property:
textBox.ForeColor = Color.Red
For a rich text box it's a bit more complicated, but you can search for substrings with that control, select them, and then set the color for the selection to red. See the documentation for details.