Convert all the newlines in a file from \r\n to \n
APL (Dyalog Unicode), 22 bytes

Anonymous tacit prefix function.
10@3∘⎕NGET⎕NPUT{⍵1}

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⎕NGET get content,encoding,line-ending from argument file
10@3∘ replace the third element (line ending) with 10, meaning \n
⎕NPUT put that into…
{⍵1} the file name followed by an overwrite flag