To import Outlook PST files into Office 365 using the Regain PST to Office 365 Migration Tool, follow these steps:
Download and install the Regain PST to Office 365 Migration Tool.
Launch the tool and add your PST file(s).
Choose the Office 365 login option and provide your credentials.
Select the mailbox or folder where you want to import the PST data.
Click on “Start Migration” to begin the import process.
Know More: https://www.regainsoftware.com/pst-to-office365-migration.html