[This](https://askubuntu.com/q/11031/743263) post explains how to add a line at the beginning of a file from the terminal. But how do I modify from the terminal a line somewhere in a file if I do not know which line it is?

I should modify the line `eni=10.*10**9` to `eni=10.*10**8` note the exponents. It is the second time that `eni` appears