If you have the GUI set to 3840 x 2160 and after an update you are having a problem where the GUI seems like it is zoomed in on the top corner of the screen or the GUI doesn’t fill the whole screen. This is likely because you have adjusted the video calibration setting in settings > system > display at some point in time, and the settings saved by doing so is confusing the new version of CoreELEC into thinking your screen is a different size then it is.
While it is possible to fix this by editing the guisettings.xml file by hand, the easiest way to fix this is to ssh into your device and run the following commands
wget https://test.coreelec.org/cdu13a/tools/guiscalefix.py -O /storage/guiscalefix.py
chmod 755 /storage/guiscalefix.py
/storage/guiscalefix.py
This will temporarily stop Kodi while the script removes the bad display settings in the guisettings.xml file , and will restart Kodi when it it done.