Could you give me some advice on the best practice to update a SQL Server 2014 instance that is on version 12.00.2000.00? Can I install the Service Packs or do I need all of the cumulative updates as well?
Install the most recent service pack, then install the latest cumulative update.
CU10 is the most recent Cumulative Update.
Ensure you take a backup of your databases onto physically different media, and do this in a non-production environment to test the change prior to doing this to your production SQL Server.