You can perform a rolling upgrade of your replica set from MongoDB 2.6 to 3.0.
While a mixed version 2.6/3.0 replica set is supported for the purposes of upgrading, you should not run with this configuration for longer than is required.
You should also read the 3.0 upgrade and compatibility change notes carefully, as important considerations will be included there. As with any upgrade, you should test in a development/staging environment before prior to your production release.
A few particular points to be aware of:
Before enabling the new WiredTiger storage engine, ensure that all replica set/sharded cluster members are running at least MongoDB version 2.6.8, and preferably version 3.0.0 or newer.
Once upgraded to MongoDB 3.0, you cannot downgrade to a version lower than 2.6.5
If you upgrade to 3.0 and have run
authSchemaUpgrade, you cannot downgrade to 2.6 without disabling
--auth or restoring a pre-upgrade backup. The
authSchemaUpgrade discards MONGODB-CR authentication credentials used in MongoDB 2.6.