Minifying can't be reversed. (There might be some interactive unminifiers that allow you to edit/copypaste/unminify/copypaste back/save individual files, but I have not really looked)
If you compile your own firmwares, the easiest is to set the LuCI build options to disable JSminify and CSStidy. Then you will have cleartext files in your router.
I do that for my own builds. Easy to edit in the live router.
But as the files are quite normal files, you can just navigate to the correct directory and edit/replace the minified file with the original from the source code repo. (I have pasted source code to text editor and saved, and also fetched the original from source repo via wget.)
Ps. Practical hint... I have notice browsers to be picky about noticing javascript changes. Sometimes I have to close the browser for it to notice the change. Similarly the uhttpd www server may cache files on router.