Hi,
You can replace the 2.5" HDD with a 2.5" SATA3 SSD such as the 2.5" Samsung 860 Evo or similar. There is no extra M2 SSD slot in the notebook.
Example:
https://www.amazon.com/Samsung-Inch-Internal-MZ-76E1T0B-AM/dp/B078DPCY3T/ref=sr_1_1?keywords=860+evo...
On page 60 of service manual you can see the steps to replace the drive:
http://h10032.www1.hp.com/ctg/Manual/c06189775
You will need to install the OS on the the SSD either by using recovery disks or HP cloud recovery
https://support.hp.com/us-en/document/c06162205
migrating an image. The manufacurerof the SSD will usually provide software (example Samsung migration software from Samsung site) and you will need a USB to SATA adapter such as this
https://www.amazon.com/StarTech-com-SATA-Drive-Adapter-Cable/dp/B00HJZJI84/ref=sr_1_1_sspa?keywords=...
Or you can perform a clean install by using Microsoft media you can download from Micrsoft
https://www.microsoft.com/en-us/software-download/windows10
Hope it helps,
David
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