uncompressing, editing and recompressing an initrd

1. Create a directory and switch into it:
   
   # mkdir test
   # cd test
2. Then uncompress and extract the initrd:
   
   # zcat /boot/initrd.img | cpio -idmv
3. Edit the contents (if needed)
4. Repack and compress the initrd image:
   
   # find . | cpio -o -c | gzip -9 > /boot/test.img
5. For image compressed with xz format, use commands below to extract the initrd image.
   
   # mkdir /tmp/initrd
   # cd /tmp/initrd
   # xz -dc < initrd.img | cpio --quiet -i --make-directories
6. Edit the contents (if needed)
7. Repack and compress the initrd image:
   
   # cd /tmp/initrd
   # find . 2>/dev/null | cpio --quiet -c -o | xz -9 --format=lzma >"initrd.img"

found on: https://access.redhat.com/solutions/24029