assuming your using uboot as your bootloader, on the device you can use fw-utils to change the uboot environment to switch between the active partitions from the operating system. Make sure you change both variables together and then reboot.
To make partition 1 the active partition:
fw_setenv mender_boot_part 1
fw_setenv mender_boot_part_hex 1
To make partition 2 the active partition:
fw_setenv mender_boot_part 2
fw_setenv mender_boot_part_hex 2
https://developer.toradex.com/knowledge-base/u-boot-fw-utils
We manufacture our products with a gpio pin connected to a physical button that allows the customer to roll back to the previous partition if they are not happy for some reason with the new update.
You can also change these variables from within a uboot session before you boot the operating system.