repmgr 4.x is implemented as a PostgreSQL extension; normally the upgrade consists of the two following steps:
Install the updated package (or compile the updated source)
In the database where the repmgr extension is installed, execute ALTER EXTENSION repmgr UPDATE.
Always check the release notes for every release as they may contain upgrade instructions particular to individual versions.
If the repmgrd daemon is in use, we recommend stopping it before upgrading repmgr.
Note that 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.