Question

please provide a comand for editcap to convert a .pcap thats actually a .pcapng file to pcap format

Answer 1

cd /path/to/folders   // the path to folder where your folders containing '*.pcapng' files are in
find . -type f -name '*.pcapng' -print0 | while IFS= read -r -d '' f; do editcap -F libpcap "$f" "${f%.pcapng}.pcap"; done

Example:
mahesh@bt:~/lab$ ls
first.pcapng
mahesh@bt:~/lab$ find . -type f -name '*.pcapng' -print0 | while IFS= read -r -d '' f; do editcap -F libpcap "$f" "${f%.pcapng}.pcap"; done
mahesh@bt:~/lab$ file something.pcap first.pcapng
first.pcap:   tcpdump capture file (little-endian) - version 2.4 (Ethernet, capture length 65535)
first.pcapng: pcap-ng capture file - version 1.0
mahesh@bt:~/lab$