Some important key for SP upgrade is :
1) SP cannot be uninstall
2) Schema will cange/extend
Below is the sequence of upgrade based on role :
1) Client Access servers (beginning with the internet-facing site)
2) Hub Transport and Edge Transport servers
3) Mailbox servers
4) Unified Messaging servers
Doing upgrade by sequence and proper step can reduce the downtime:
CAS array
1) Drainstop
2) Stop conflicting services
3) Disable monitoring
DAG Member
1) Failover
2) Stop conflicting services
3) Disable monitoring
Reference :
1- Full instruction for upgrade Exchange to SP3
2- CAS Array update instruction
3- DAG Member update instruction