If your MOX A is connected to the internet you can insert your microSD card into the microSD slot on the main board of module A and choose rescue mode number 6 when staring the MOX. MOX will download the image from the internet and use it.
Other option is to prepare your microSD card manually. If you have a Linux PC, you can do that from CLI as a root using following commands (assuming your SD card has one partiotion and your SD card reader is
mkfs.btrfs -f -L turris /dev/sdx1 mkdir -p /mnt/turris mount /dev/sdx1 /mnt/turris btrfs subvol create /mnt/turris/@ wget -O - "https://repo.turris.cz/hbs/medkit/mox-medkit-latest.tar.gz" | \ tar -C /mnt/turris/@ -xzvf - btrfs subvol snapshot /mnt/turris/@ /mnt/turris/@factory ln -s @/boot/boot-btrfs.scr /mnt/turris/boot.scr