How to change Android system boot slot form A to B

Changing Android's Active Partition Slot fastboot get active slot It may not flash TWRP as there is no active slot not bootable) If it is a slotted device in Fastboot run the command: fastboot getvar fastboot set active slot 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 do

fastboot get active slot Boot your smartphone into Fastboot Mode · Now, connect your smartphone to the PC using a USB cable · Open a Command Prompt, Terminal, or