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That's trivial. Read the line, change the data in the line, write the line back to the file. No appending or any other tricks are required. Try printing (to the screen) the line as it is read from...

And there's only one line of data in the whole file?

It occurred to me after reading Norm's message that you may not have explained the problem thoroughly. If you want to intersperse new data with old data in the lines of the file, like:

OLD
new...

Investigate (search) how to open the file or the mechanism that writes to it by appending data rather than writing to it replacing the existing data.