You need to upload the new photopost files to your photopost directory on the server. Upgrade documentation for that product is in the documentation folder of the downloaded code. Nothing changes on your existing gallery the new files along with upgrade script upgrade you to the latest.
you most likely need to ask your host the path you can try using /usr/local/bin/mogrify or /usr/bin/mogrify but your host would know if its installed or not. For you to find out you need full ssh access to the server.