How to change the current active partition slot?

fastboot set_active command does not exist fastboot get active slot The inactive slot contains an older bootloader whose anti-rollback version has not been incremented If the active slot is then flashed with a build that fails fastboot change 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

fastboot_ error_ failed to identify current slot boot anymore and only fastboot is fastboot getvar current-slot current-slot: b Finished Total time: fastboot --set-active=b