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You can Merge PST files by performing the task within two processes;
1: Launch MS Outlook application on your computer.
2: Click on Home menu-> Select New Items -> Press on More Items options-> and then Click on Outlook DataFile.
3: In the Create or Open Outlook Data File window, choose a location to save the new PST file-> Click OK.
Once the file creation is done, you can select the Import option of MS Outlook for importing all the PST files one by one into the new PST file. Follow the Import PST File option using below few easy steps;
1: Click on File menu-> Press on Open tab-> Choose Import option.
2: In the Import and Export Wizard dialog box, click on Import from another program or file from the Options-> and then Press on the Next button.
3: Now, in the Import a file window, Click on Outlook Data File (.pst)-> and then Tap on Next button.
4: After that, click on the Browse button to choose the PST file that you would like to import into a new PST file. Also, choose other options (if needed). Click on the Next button.
5: Now, choose the mailbox folder (of the selected PST file) that you wish to import into a new PST file. Additionally, you can also choose other options (if required)-> Click on the Finish button.
6: Once the process will start, the importing PST files into the newly created PST files begins.
I hope this helps.