Upgrading NBU 7.5.0.6 to 7.6.0.2 in cluster environment

Hi,

We are planning to upgrade our netbackup version from 7.5.0.6 to 7.6.0.2. I need below information

1 do we need to upgrade all media servers on same time along with master server upgrade. or it could be done afterwards, in phases

2 how long will it take for database rebuild, and it depends on what parameters

3 If anyone has done the same in clustered environment, please share the step by step method with all pre-requisites.

Thanks

Manish