There should be a definite plan to implement a cluster upgrade,and its important to have a proper checklist for the same:
1. Take database backup’s including system databases.
2. Script out all the users.
3.Script out all the jobs and other objects if any.
4. Ensure that required .Net framework and windows installer is installed for both nodes,this will ensure that unexpected downtime is avoided.
4.Start upgrade process from the passive node.
5.Once the passive node is upgraded,failover to the upgraded node and proceed with active node upgrade.
6.Once both nodes are upgraded,ensure that proper failover and failback happens between nodes.