This images include proprietary or open source drivers for DVB cards which are incompatible to the current kernel or couldn't be included due disturbance with the kernel drivers (all drivers in one image).
This isn't an automatic process - you have to choose in the DVB Drivers Configuration Addon which driver you want to use.
You can choose the wrong driver without consequences (besides that the DVB card is not working). Just select the driver you want to try and select him.
After the selecting the system will reboot and your driver is now ready to use (you need nothing to do besides that).
Disable auto update at the LE-Config Addon - or you end up at an official LE image after reboot :)
Update from LE:
You can update from an existing LibreELEC installation (also update back to an official image) without loosing your data. Just drop the *.tar or *.img.gz image to your update folder (at the network share or via ssh).
Update from OE:
Update from OE works (most of the times) but remember OE only support *.tar to update. MAKE AN BACKUP!