How to change the current active partition slot?

I am trying to root my Xiaom mi a2 phone But i am stuck on changing the active partition When i enter fastboot set_active b it doesnt

Card slot microSDXC Internal 32GB 3GB RAM, 32GB 4GB RAM, 64GB 4GB RAM, 128GB 4GB RAM eMMC MAIN CAMERA Camera 48 MP, f, 26mm (

change slot Fast boot times Ability to create or change themes Includes Retroarch PortMaster Cons Less frequent updates Linux style menu Currently in Alpha KNULLI

fu dao le slot By default, fastboot flashes the current slot on an AB device If the update package also contains images for the other, non-current slot,