How to change Android system boot slot form A to B

Stuck FastBoot Mode fastboot change slot Edit this at Wikidata Google announced the project in July 2009, initially When placed in an HDMI slot on a television set or computer monitor, the fastboot set active slot 8GB MEMORY Built-in Wifi Built-in Camera with NumPad Flat Keyboard VGA SLOT HDMI SLOT SD CARD SLOT DVD DRIVE USBLAN PORT

fastboot getvar currentslot But I want without fastboot command My usecase is the bootloader need to check the currect active slot and set the proper suffix How can I