how to write a line in a text file

[nephish at xit.net]

Hey there,
kinda newbie question here.
i know how to read the lines of a txt file.
i know how to write a txt file.

but how do i overwrite a line value with another value ?

i mean, how do go to, say, line 3 of a text file and replace
what is written on line 3 with something else?

thanks
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