repmgr is updated regularly with point releases (e.g. 4.0.1 to 4.0.2) containing bugfixes and other minor improvements. Any substantial new functionality will be included in a feature release (e.g. 4.0.x to 4.1.x).
repmgr is implemented as a PostgreSQL extension; to upgrade it, first install the updated package (or compile the updated source), then in the database where the repmgr extension is installed, execute ALTER EXTENSION repmgr UPDATE.
If repmgrd is running, it may be necessary to restart the PostgreSQL server if the upgrade contains changes to the shared object file used by repmgrd; check the release notes for details.
Please check the release notes for every release as they may contain upgrade instructions particular to individual versions.