Below are 2 links. The first one is the official Teclast website. On this page you can enter the product ID (E5A3) into the box and press enter. It will then take you to the second link. There you will find the image file. You must download all 5 parts and decompress it to a new folder on your computer. The second link is actually where the files are stored (Microsoft Onedrive cloud where Teclast store the images).
You can try going directly to the second link. If it fails then use the first link as explained above.
I should have mentioned with the first link, on the orange menu bar at the top of the page change the language to your own and then click on <span style=”color: #ff0000;”>Download -> Firmware</span>
<span style=”line-height: 1.5; color: #ff0000;”>I strongly suggest using the second link</span>