Can you legally change your credit file?

Yes.

OK let's be a little more specific. If you believe there is an inaccuracy on your credit report, then you can notify the agency that the reporting entity needs to prove this item. Then they have a time period to validate the item (I think it's 30 days). If they do not within the alotted time, then the credit bureau will remove the item from your file.

Technically, you could challenge every 'negative' item on your credit report (with all 3 agencies), and chances are, a lot of the reporting entities would not get back with proof of validity within the timeframe. Even though the item may be legitimate.

While this practice is technically legal, it is certainly not ethical.