EDIT: Don’t use this image. Otherwise you will only get 3GB of total space in your /storage directory. Instead use the ceemmc tool (you’ll get about 25GB this way).
First make a backup. Then either download it, or store it on a USB drive. You will need this later.
Then go here, hit the 0.5.4 firmware.
Specifically you want what’s in 0.5.4->DualBoot CoreELEC
Grab the zip file without ota in the name.
There should be an Amlogic Firmware tool you will need to install to proceed. The img file after extraction is about 1.9GB (if it’s 1.4GB it’s the wrong one). Process takes around 5 minutes. Oh and you need a USB-A to USB-A 3.0 OTG cable. First boot up will take you to the regular Ugoos image. Restart into CoreELEC. It’ll be an older version so it’ll ask to update a couple of times. Then restore the backup.