I assume this is referring to the changes outlined here - Outlook for iOS and Android in Exchange Online.
As I understand it, it's not a setting as such but more an architectural change that will affect the whole tenant and streamline how Outlook mobile apps communicate with Exchange Online.
This new native Microsoft sync technology improves and simplifies data synchronization, offering a number of benefits, mostly improving performance and robustness. There is a FAQ here that answers some of the questions about this - Native Microsoft sync technology migration including the answer to your main question:
As a tenant administrator, can I control which of my users will be migrated to the native Microsoft sync technology?
No, the migration to the native Microsoft sync technology will be on a tenant-by-tenant basis and not a per-user basis. While the tenant selection order for migration is random, we are being deliberate about migrating Office 365 mailboxes first. If you are a customer operating in a hybrid configuration where a portion of your mailboxes remain on-premises, the on-premises users leveraging hybrid modern authentication will be migrated to the native Microsoft sync technology at a later date. This means that your Office 365 users will migrate to the native Microsoft sync technology, while the on-premises users continue to use the REST API to connect to Exchange Online.
Once your tenant is migrated, a user will not switch to the native Microsoft sync technology, until after they launch/resume Outlook for iOS and Android.
Hope that helps!
