how to recover items in recoverable items folder?

As the end user, or as an admin? For the former: https://support.office.com/en-us/article/recover-deleted-items-in-outlook-for-windows-49e81f3c-c8f4-4426-a0b9-c0fd751d48ce

For the latter:

https://blogs.technet.microsoft.com/recoverableitemscmdlet/2018/01/08/45/