# Move the app files from the old volume to the desired one.
sudo mv /volume1/@appstore/[app_name] /volume3/@appstore/

# Delete the obsolete Symlink pointing at the old app path
sudo rm -fv /var/packages/[app_name]/target

# Create a new Sim link pointing at the new, correct app path.
sudo ln -s /volume3/@appstore/[app_name] /var/packages/[app_name]/target

Full article: https://www.mcleanit.ca/blog/synology-move-application-volumes/