SAMBA+ consists of numerous packages. Which packages you need depends on the server role you are going to run your Samba server in. A list of all packages and the corresponding description is available in the repository meta data (e.g. https://download.sernet.de/subscriptions/samba/4.18/rhel/8/repodata/ for RHEL 8).
Additionally, you can show the package information with the packet manager commands, e.g. 'apt show sernet-samba' on Debian/Ubuntu.
There are regular major and minor upstream Samba releases. (For details on upstream Samba release cycles and currently supported branches, please see the Samba Release Planning.)
Please make sure that your running Samba version is still maintained. Regular updates are highly recommended for security reasons. SerNet usually recommends to run the "Maintenance Mode" Samba on production servers. Please note that manual interaction is needed to stick to the "Maintenance Mode" (see "Upgrading SAMBA+" below)! That means, every 6 month, you need to adapt the version number to the current "Maintenance Mode" in the repository files. Otherwise, the systems will end up running an unsupported Samba version, which won't receive any security updates after ~12 month.
Minor update means, the first and the second part of the version number does not change (e.g. 4.13.1 -> 4.13.2). To update to a newer minor version, you can just use your system's package management system to install the new package updates. It makes perfect sense to read the release notes of the new version before updating.
Upgrading means to go to a new major release (the first and/or second part of the version number changes). Testing the upgrade of your individual Samba setup in a test environment is highly recommended! Please read at least the first release notes of each new major version (e.g. when you are upgrading from Samba 4.10.4 to 4.13.2, read at least the release notes of Samba 4.11.0, 4.12.0 and 4.13.0). Especially the section "UPGRADING" is very important. Also running 'testparm' after upgrading is recommended, which complains about deprecated or already removed options in your Samba configuration.
To upgrade, you need to manually adopt the repository files to switch to the new repository path (e.g. 4.12 -> 4.13). This is not done automatically as it must be an active decision of the administrator. After that you can refresh your repository and proceed with the installation of the new version as normal.