I recommend to read the Getting Started Guide
first to get a feeling how docx4j works in general.
Then come back a post a more precise question, including information about your source document and what you want to achive.
P.S.: "Normal" text kann be found in a Paragraph (P), which contains a Run (R) and then the Text. A Text Object has a methode setValue(String s), which can be used to alter the text in it.