Scribus can read PDF files but is not a PDF editor.
It's not the right tool for tweaking little bits of text in a PDF.
Also, PDF is not a format that is meant to be further edited.
But, if I had to modify one, I would first try Inkscape.
Then LibreOffice Writer / Draw.
In the past I've used flpsed, but i'm not sure I would still use it.
That being said:
If you're using a stable Scribus 1.4.x, all you can do is:
- load the PDF as a background image on a page (put a full page image frame behind everything),
- cover the old text with a white rectangle and
- retype the text.
Slightly better than tipexing on paper.
If you're working with a development Scribus 1.5.x (or later)
- you can load the PDF page as a vector,
- remove the old text and
- type the new text.
But
- The text will always be vectorized on import (no real editing is possible) and
- it won't reflow.