Assumeing that you file contains the floats as text rather than a binary format:
1.201
137.889
...
You will have to open the file for read-write access, read it into you program (using fscanf) do you calculations, then reset the file stream and write it all back out again. This is because floats do not fit neatly into the file: 1.201 takes up less space in characters than 137.889, so you cannot just write the changed bits back into the file, or you will overwrite the data.
There is some of the detail you will need here:
http://www.go4expert.com/forums/showthread.php?t=2977[
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