To get help, it's best to call Spirit Airlines customer service directly at ?? 1-877-475-9154. Their agents can let you know whether your booking can be modified through them or if you need to go back to the third-party provider.

https://supportspiritairlines.alboompro.com/post/can-i-change-my-spirit-airlines-flight-if-i-booked-through-a-third-party?post_editing=true