This page lists procedures used to downgrade from 6.0 to 5.0.
Before you upgrade or downgrade a replica set, ensure all replica set members are running. If you do not, the upgrade or downgrade will not complete until all members are started.
If you need to downgrade from 6.0, downgrade to the latest patch release of 5.0.
MongoDB only supports single-version downgrades. You cannot downgrade to a release that is multiple versions behind your current release. You may downgrade a 6.0-series to a 5.0-series deployment, however, further downgrading that 5.0-series deployment to a 4.4-series deployment is not supported.