If I understand correctly, you should be able to use this. The in clause will return all of the cxa_adr_key values that currently have cxa_on_hold_flag = 1. The update statement will then update all of the cxa_on_hold_flag = 1 for those keys. Does this look like what you want?

update co_customer_x_address

set cxa_on_hold_flag = '1'

Where cxa_adr_key IN (Select cxa_adr_key WHERE cxa_on_hold_flag = 1)