It is very easy to replace an image file for a photo that you've already added to a gallery.
The simplest way is to modify the photo via PHOTOS > MODIFY. On the photo modification page you can select and upload a new image to replace the existing one.
Alternatively, you can use the PHOTOS > REPLACE IMAGE page, where you can upload multiple replacement images at once. In this case, make sure that the new files have the EXACT same filenames as before. As long as the filenames are exactly the same as the existing ones, the new images will overwrite the old ones, and you don't have to mess with the database information at all.
Replacing an image will not change or remove its data; it only replaces the image file itself.
DO NOT delete and re-add an image from the galleries in order to update the image file. This is unnecessary and more tedious because you'll have to re-enter all the photo data, print assignments, etc.; and also it will potentially change the web address for the photo's page, which is not ideal.
