This post explains how to change the file behind an existing link, if you have edit privileges.
Go to the page concerned and select Edit in the footer
Position your cursor at the beginning of the link/file to be updated
Scroll up and select the Upload/Insert button
Find, and select, the replacement file from your computer
Adjust the title to what you want the link to read
Click the “Insert into Post” button at the bottom of the screen.
Highlight the old link and delete it.
When satisfied with the result, click the Publish button on the right of the screen