how do i change hyperlinks inside pdf using python?

So I couldn't get what you want using the pyPDF2 library.

I did however get something working with another library: pdfrw. This installed fine for me using pip in Python 3.6:

pip install pdfrw

Note: for the following I have been using this example pdf I found online which contains multiple links. Your mileage may vary with this.

import pdfrw

pdf = pdfrw.PdfReader("pdf.pdf")  # Load the pdf
new_pdf = pdfrw.PdfWriter()  # Create an empty pdf

for page in pdf.pages:  # Go through the pages

    # Links are in Annots, but some pages don't have links so Annots returns None
    for annot in page.Annots or []:

        old_url = annot.A.URI

        # >Here you put logic for replacing the URLs<
        
        # Use the PdfString object to do the encoding for us
        # Note the brackets around the URL here
        new_url = pdfrw.objects.pdfstring.PdfString("(http://www.google.com)")

        # Override the URL with ours
        annot.A.URI = new_url

    new_pdf.addpage(page)    

new_pdf.write("new.pdf")

I managed to get it working with PyPDF2.

If you just want to remove all annotations for a page, you just have to do:

if '/Annots' in page: del page['/Annots']

Else, here is how you change each link:

import PyPDF2

new_link = "https://www.youtube.com/watch?v=dQw4w9WgXcQ" # great video by the way

pdf_reader = PyPDF2.PdfFileReader("input.pdf")
pdf_writer = PyPDF2.PdfFileWriter()

for i in range(pdf_reader.getNumPages()):
    page = pdf_reader.getPage(i)
    
    if '/Annots' not in page: continue
    for annot in page['/Annots']:
        annot_obj = annot.getObject()
        if '/A' not in annot_obj: continue  # not a link
        # you have to wrap the key and value with a TextStringObject:
        key   = PyPDF2.generic.TextStringObject("/URI")
        value = PyPDF2.generic.TextStringObject(new_link)
        annot_obj['/A'][key] = value
    
    pdf_writer.addPage(page)

with open('output.pdf', 'wb') as f:
    pdf_writer.write(f)

An equivalent one-liner for a given page index i and annotation index j would be:

pdf_reader.getPage(i)['/Annots'][j].getObject()['/A'][PyPDF2.generic.TextStringObject("/URI")] = PyPDF2.generic.TextStringObject(new_link)