Updating nginx should not affect any config files that you have created or modified. You can update nginx to the latest version by running
sudo apt-get update to update the package lists, followed by
sudo apt-get install nginx.
If you want to be extra careful, you can back up the
/etc/nginx directory which contains all of nginx's config files.
sudo cp -r /etc/nginx /etc/nginx.pre-upgrade
After upgrading the package, you can delete
/etc/nginx and move
/etc/nginx.pre-upgrade back to
/etc/nginx. However, as I said, it is unnecessary and completely optional to do it.