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.

If you don't like the previous option, you can just download the image and save it (as it is, no unpacking) to your microSD card or any USB drive and use rescue mode number 4.

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 /dev/sdx)

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