AllyBri- I just did the same thing this morning for my friend - I have been doing the planning for her and noticed that her return flight fares had dropped - I called Spirit and got a very nice woman who asked if I was on the computer while talking to her - I said yes, so she directed me to the home page and walked me through the entire process. Very easy and now my friend has a voucher attached to the same reservation ID for future travel within one year of the original booking date.
If you can be at the computer and on the phone at the same time, call and explain your difficulty and ask if they will help guide you through the process. Good luck!