Open the PDF in a PDF editor that supports text editing
Select the text you want to change
Edit the text directly if the file is editable
Save the updated PDF
If the PDF is scanned, run OCR first
Use OCR to convert the scanned pages into editable text
Make the text changes after OCR
Save the edited file as a new PDF
If you do not have a PDF editor, convert the PDF to Word or another editable format
Edit the text in the converted file
Export or save it back to PDF
Check the final PDF for formatting issues
Use a tool like Adobe Acrobat, Foxit PDF Editor, or similar software