Open the PDF in a PDF editor that supports editing
Use the text editing tool to select the existing text
Delete or replace the text you want to change
Add new text in the same area
Save the edited PDF as a new file or overwrite the original
If the PDF is scanned, run OCR first to make the text editable
If needed, use annotation tools to cover old text with a white box and add new text on top
Flatten the PDF after editing if you want to prevent further changes