on most laptops only one M.2 slot is pcie speeds the other is sata so thats something to consider, I looked on the lenovo website for your laptop and i think yours is like that ( https://www.lenovo.com/au/en/laptops/thinkpad/thinkpad-x/ThinkPad-X1-Carbon-6th-Gen/p/22TP2TXX16G its under techspecs) so check what speed you current ssd, if its pcie speed I would get a 1tb samsung 860 evo, if its sata speeds get what ever 970evo you can afford