"At first read the system requirement and compatibility for SQL Server 2014 and check whether your current server configuration meets the requirement or not.

Before upgrading the SQL Server, please take entire/full back up of the current Active/Passive Cluster instance(s) and then try to update one of the instances and then go for another.

Please see the below link for more clear understanding

http://logicalread.solarwinds.com/what-sql-server-clustering-can-and-cannot-do-w02/#.V3EWgfl96Uk"