Hello Padmanabh,
Welcome to Microsoft Community.
Unfortunately, your requirements are not available, as the SSD of the Surface Pro 6 is not removable.
If you are going to remove the device and replace the 120GB SSD with a 500GB SSD, I highly recommend not doing this, as it is extremely risky and will most likely cause the Surface to fail to boot after the SSD has been replaced.
If it's just for more storage, I actually recommend using an SD card, because the Surface Pro 6 comes with a microSD™ card reader:
Installing a 1TB SD card and then using the Surface's 120GB SSD as a Windows 10 system disk is a more reasonable suggestion.
Best Regards,
Mitchell | Microsoft Community Support Specialist