Hi,
At the moment you have a 128GB M2 SATA3 SSD in the system. So you can replace with a M2 SATA3 SSD of your desired capacity. One of the best would be the M2 Samsung 860 Evo or similar.
Example:
On pages 24-27 of service manual, you can see the steps to remove the top cover and then replace the drive.
You will need to reinstall the OS on the SSD either by using recovery disk, migrating an image or performing a clean install with Microsoft media you can create at Microsoft support page:
https://www.microsoft.com/en-us/software-download/windows10
Hope it helps,
David
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