Updating CU5 from CU3 any known issues?

Hi All, Planning to update CU5 2013 for our servers (Currently Running 2013 CU3)

we have 1 DAG, 3 members (2 Members in Primary Site & 1 in Another Site )

Is there a best approach steps available to update 

Note:

1. We have customized out OWA page - Will it retain? 

2. What other areas of concerns while going for the update

Manju Gowda