The command for the manual update procedure can be found here: Outlook will automatically download updates only unless the size of the changes is larger than 12.5% (one-eighth) of the total OAB size.
In that case the whole OAB is being downloaded again.
It contains all the email address and contact information of an individual profile for Outlook.
Sometimes, users have faced an issue like “Outlook Offline Address Book not updating” because of resources available on the host server and service failed.
In general, Outlook will automatically update the Offline Address Book (OAB) for Exchange accounts once every 24 hours.
When it didn’t or when you want to make sure you do have the latest OAB available, you could manually trigger an update from within Outlook as well.
The Outlook Client failed to download an updated copy of the Offline Address Book because the services have stopped functioning.