Back up your email into a .pst file
Select File > Open & Export > Import/Export.
Select Export to a file, and then select Next.
Select Outlook Data File (.pst), and select Next.
Select the mail folder you want to back up and select Next.
Choose a location and name for your backup file, and then select Finish.
If you want to ensure no one has access to your files, enter and confirm a password, and then select OK.
Import Outlook items from a .pst file in Outlook for PC
At the top of your Outlook ribbon, select File.
If your ribbon doesn't have a File option in the top left corner, you aren't using an Outlook app installed on your computer. See What version of Outlook do I have? to find your version of Outlook and to get to right import steps.
Select Open & Export > Import/Export. This starts the Import/Export wizard.
Select Import from another program or file, and then click Next.
Select Outlook Data File (.pst) and click Next.
Browse to the .pst file you want to import. Under Options, choose how you want to deal with emails and contacts, then choose Next.
If a password was assigned to the Outlook Data File (.pst), enter the password, and then click OK.
If you're importing the contents of your .pst file into your Office 365 mailbox, choose that mailbox here.
If you're NOT importing into your Office 365 mailbox, choose Import items into the current folder. This imports the data into the folder currently selected.
Choose Finish. Outlook starts importing the contents of your .pst file immediately. When the progress box disappears, the import is finished.