oh wow talk about luck, i had the same dongle as the OP and have spent a few days looking for how to update the dongle w/ a new payload. Back and forth conversation with the seller and they ended up giving me a refund. And I didn't realize that it has a sd card slot until I read this post!
Turns out updating the dongle is very simple!
1. download the payload of your choice and rename the payload to "payload.bin"
2. copy "payload.bin" to the root of the sd card
3. put the sd card in the dongle
4. enter rcm mode
5. plug in dongle, you'll notice that the red light flashes much longer
6. once you have booted you can take out the sd card, the dongle is now updated w/ your new payload and no longer require to be present in the dongle.
Since it comes default with ReiNX, I put Hekate 4.2 in and verified that it does boot hekate. Removed the sd card from the dongle and it still boot with Hekate this confirms that the dongle is updated.
Now if you put the sd card into your pc, you'll notice that "payload.bin" is removed from the sd card.
Hope this helps!