You just upload the new pdf (or other file type) to media then link it to the delivery page and delete your old link. Don’t forget to update (save) the page.
If you want to add some extra bonus material you can also just add it to the page directly.
- load the file in your media library
- go to edit the page and place your cursor where you would like the link to appear on the page
- click the Add Media button above the editor window
- select the file you want to add
- type in the title you want to display on the page
- click the Insert Into Page button
- update page