uImage.initramfs

Hello, i have system using openWrt uImage-Files. Here i have to change the network-Settings, but i do not have access to the device if it is online. Now i thought i can change the networkconfig in uImage.initramfs-File. I could exctract the file und found the file for changeing, but i do not know how i make a initramfs file.
Could s.o. please help me?
Thanks a lot best regards
Tobias