How can I reinstate my fiances legal residency status?

If you are a US citizen, she entered the US with the intention of leaving on time but overstayed her admission period and you marry, you can file for a green card for her in the US. Given that she is out of status, you should meet with a competent immigration attorney in your jurisdiction to determine how your local DHS office handles that type of case. That attorney can also analyze all other factors of your case-finances, prior marriages, immigration history, etc to verify that there are no issues barring the green card application.

Your question turns on several factors. I presume that since you called your girlfriend your fiance, that you will be marrying her. After your marriage, you can file to adjust status if she entered legally and does not fall under certain limitations that are specific to certain visas. The adjustment of status application process is quite involved and includes the filing of numerous forms and supporting documents. Just because you may satisfy the requirements to file for an adjustment, does not mean that the petition will be approved. The approval decision is reached after the weighing of several factors, including but not limited to, criminal history, the validity of the marriage, the evidence supporting the petition, as well as other case specific information. I would encourage you to speak with immigration counsel to determine where you stand in your case.