Thx a lot.Just use the way it is described in post #3
1. Download the flashable *.zip file for an f-Version you would like to flash and put it on your phone
2. Boot into TWRP recovery
3. Settings -> disable / uncheck "Zip siganture verification" under settings
4. Install -> select *.zip from the palace you put it in step 1 and install
5. Reboot