How to unlock read-only filesystem on OpenWRT router linux distribution?

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http://unix.stackexchange.com – I have installed minimal OpenWRT firmware on Linksys router. When I try to modify some of the files withing /etc/ directory for example firewall.user file by using command vi firewall.user it opens file in read-only attributes only. How can I enable editing of files? The chmod command isn't accessible and my memory in router is very limited, therefore installing any extra resources is not an option. (HowTos)