How could I fix my girlfriend's immigration papers?

I do not know. Are you a US citizen? How old is your girlfriend and how old was she when she entered the US? If needed, you should be prepared to file for a waiver, but start with meeting with an attorney.

Best wishes with your upcoming marriage plans. Simply becoming married does not change anyone's immigration status, but frequently a marriage will create eligibility for one spouse to file an immigration application on behalf of the foreign national spouse. Issues that may address eligibility for a marriage-based "Immediate Relative" visa application include such things as: your citizenship; your fianc?e's current immigration status, if any; details about how and when your fianc?e entered the U.S.; etc. Especially since you appear to be less than thoroughly familiar with the immigration application process, it would be wise for you and your fianc?e to engage an immigration attorney who, after learning all of the relevant details, could advise you about immigration-related eligibilities, options and strategies and represent you in the application process. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.