Best way, on my laptop is:
sudo prime-select next-boot intel2
for intel
sudo prime-select next-boot nvidia
for nvidia gpu
reboot is saver i think.
with
sudo prime-select next-boot intel
i get stuck in black screen with blinking coursor. therefor i use intel2 and it comes up
if i stuck in black screen, i use ctrl + alt + F2, and login to switch.