from this forum I know that I have to update my WinTV Solo HD firmware to watch DVB-T2 correctly (the channels of one tvheadend mux are disturbed):
DVB-T2 ZDF Mux in Leipzig/Germany at 482MHz not working - Tvheadend
When I try to exchange the file dvb-demod-si2168-b40-01.fw on my Raspberry 4, I cannot because it's a read-only filesystem. I tried to remount root and I tried to mount the sd card on my Laptop. Neither helped.
So how can I update the firmware file on my Raspberry Pi 4?