For SOME model of HP , DELL , Toshiba etc. there are separate GPU card
It`s mounted on the Laptop motherboard by the mini PCE slot so you can RAPLACE or Upgrade it.
But the options in this case is 1-2 models of GPU so the improvement is not 100% ....
The second problem is that You cant upgrade 40 -50 W GPU with 250 W new GPU on the Laptop
They have 90 % passive cooling and 10% active cooling so you cant have such high end GPU in Standart Low & Mid range Laptops
The only option is to look for High End one : AlienWare , Asus , HP , DELL
- there are even 2 x GPU Laptops but they are XXXX M - mobile versions all .......
like tis one ...