The latest Microsoft support documentation can be found here.
Get access to and back up a former user's data
12 January, 2018
When an employee leaves your organization, you probably want to access their data - meaning their documents and emails - and either review it, back it up, or transfer ownership to a new employee.
If you only remove a user's license but don't delete the account, the content in the user's OneDrive will remain accessible to you even after 30 days.
Before you delete the account, you should move the content of their OneDrive to another location that's easy for you to access. If you already deleted their account, you have 30 days to restore it. At that point, the data will be available to you even after 30 days. However, if you don't restore the account, after 30 days the OneDrive content is permanently deleted.
To save the content, here's what you do:
To preserve a former employee’s OneDrive for Business documents you 1) access the former employee’s OneDrive for Business, and then 2) move the files.
Get access to the former employee’s OneDrive for Business documents
Sign in to Office 365 with your work or school account.
Go to the Office 365 admin center.
Go to Active users and select the user.
Expand OneDrive Settings in the user details pane, and then click Access files.
Copy the files to your own OneDrive for Business or a common location.
There are a few ways to copy files in Office 365. See Video: Set up document storage and sharing in Office 365. Or, to sync your files, and then upload those files to your OneDrive for Business or your team site, see Sync files with the new OneDrive sync client in Windows.