If minimizing downtime is a goal here, then you might want to investigate converting the standby to a Transient Logical Standby to support a rolling upgrade. Metalink Note 949322.1 has more details. That note references this white paper which you can read even if you do not have MOS access:
Thanks for the response, We want to check if the mentioned steps are possible or not. Or if some recommend something else with tehe same architecture.