Under "Settings > System > Advanced > System updates" select local update from the three dots on the top right side. From the file manager locate and select the zip file as described in the instructions under the Spoiler "Manuelle Installation". No unpacking required (else the guide would mention it). After the selection you still need to start the installation.
After the installation do not reboot but perform a factory reset as mentioned in the Spoiler "Wechsel zwischen ShiftOS-G und ShiftOS-L".
After the installation do not reboot but perform a factory reset as mentioned in the Spoiler "Wechsel zwischen ShiftOS-G und ShiftOS-L".