I've found it's particularly import to not jump MongoDB versions when upgrading.
Use the upgrade notes on each version's Release Notes
The basics of upgrading are this:
Verify the running version
mongod --versionCheck the Feature Compatibility Version. The version shown here should be the same as what is currently installed.
mongo
use admin
db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )Stop MongoDB
systemctl stop mongod.service
systemctl status mongod.serviceChange the version number on the APT repo (consult the MongoDB website for how to correctly setup their repo)
#deb http://repo.mongodb.org/apt/debian buster/mongodb-org/5.0 main
deb http://repo.mongodb.org/apt/debian bullseye/mongodb-org/6.0 mainInstall the new version
apt update
apt upgradeEnter into the MongoDB Shell
mongoshSelect the admin database
use adminSet the Feature Compatibility Version for the new version
db.adminCommand( { setFeatureCompatibilityVersion: "6.0" } )Restart MongoDB to ensure all changes are applied
systemctl stop mongod.service
systemctl status mongod.service