Important
These instructions apply to self-managed deployments. To change the MongoDB version on an MongoDB Atlas cluster, see Change the MongoDB version. If you manage your deployment with Ops Manager or Cloud Manager, change the MongoDB version using the application's upgrade procedures instead.
Important
Feature Compatibility Version
To upgrade to MongoDB 9.0 from a 8.0 deployment, the 8.0 deployment must have featureCompatibilityVersion set to 8.0. To check the version:
db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )
Upgrade Path
Important
Before you upgrade or downgrade a cluster, ensure all cluster members are running. If you do not, the upgrade or downgrade will not complete until all members are started.
For any MongoDB release, you can upgrade or downgrade to the immediately previous major or minor version.