You can update using the normal update method.
HOW TO:Update LibreELEC - LibreELEC it's the same for all LE images.
It is/was the same for openelec as well. The only difference between LE and OE is, that OE is only able to handle the *.tar file while LE is able to handle both (*.tar and *.img.gz) for the update process.
I always thought I had to extract the contents of the TARGET folder from the image file into the update folder, I'm sure it is described somewhere as the proper way. This could have saved some time... Oh well 
*.tar files never had to be extracted. You have to extract the img.gz file for the openelec installation. But with the correct writing software, you even dont have to do that.
*.tar files never had to be extracted.
That's not true 
They had to be extracted in pre OpenELEC 6.0 days where you had to put the SYSTEM, SYSTEM.md5, KERNEL and KERNEL.md5 files in the update folder.
really? Damn....didn´t know that 
So yeah, my bad then. 
