On the image/file there should be a section that says "People & Things in the Image: "
Underneath the image name is a list of radio button choices - one of which is "Remove from image"
Click the profile, scroll down and SAVE.
If the image is attached to only one profile, it will be removed, although sometimes it may "hang around" for a while, until the database realises you really want it gone. (If it is attached to multiple profiles (as in my screencap), then you need to click the radio button for all.)
You then upload the new image (source/pdf/whatever) and attach it as usual.