If you're just adding a storage drive, you won't need to reinstall Windows. Follow directions in the XPS 15 9520 Service Manual.  Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. I know many people do such work without this, but it's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.
Hi, do I have to install Windows again after installing 2nd SSD in slot 2 leaving slot 1 or the bootable SSD untouched? (DELL XPS 15 9520)
Upgrading SSD in XPS 9520 and windows installation