To upgrade an existing MongoDB deployment to 3.0, you must be running 2.6. If you’re running a version of MongoDB before 2.6, you must upgrade to 2.6 before upgrading to 3.0. See Upgrade MongoDB to 2.6 for the procedure to upgrade from 2.4 to 2.6. Once upgraded to MongoDB 2.6, you cannot downgrade to any version earlier than MongoDB 2.4.
One of the most important thing to remember after you are on 2.6 if you are trying to use WiredTiger as your storage engine, you do need to take a mongodump by connecting into current mongodb version and then switch to mongodb 3.0 with a different db path,
IMP: You will not be able to use existing DB path as previous storage on mmapv1
Command to use: mongod --storageEngine wiredTiger --dbpath
mongodump and mongorestore can operate against a running mongod process, and used to take a backup and restore the backup respectively.
New in version 3.0.0.
Specifies the storage engine for the mongod database.
Valid options include mmapv1 and wiredTiger.
If you attempt to start a mongod with a storage.dbPath
that contains data files produced by a storage engine other
than the one specified by storage.engine,
mongod will refuse to start.