What part are you trying to change?
To get the twine holder out you will need to remove the twine discs first. To do that you need to drive the hidden roll pin from it's drive gear... Then remove the twine discs. Then the holder will probably come off. It looks like there is a shoulder on the pin that prevents it from falling out of the holder.... so if it's loose in the knotter frame it should just come out once the discs are gone.