How to change Android system boot slot form A to B

i do the fastboot with command my phone reboot : black screen with only and stays like that : => FASTBOOT mode then i command fastboot

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

change slot All file download from MiFirm is Free Fastboot firmware, ZIP firmware is official from Xiaomi EU ROM, TWRP and Other files is unofficial Use at your own risk

zeed24h slot Card slot microSDXC Internal 32GB 3GB RAM, 32GB 4GB RAM, 64GB 4GB RAM, 128GB 4GB RAM eMMC MAIN CAMERA Camera 48 MP, f, 26mm (