At the top of your Outlook ribbon, select File.
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.
Choose Finish. Outlook starts importing the contents of your .pst file immediately. When the progress box disappears, the import is finished.