Is your CFI expired and you need it reinstated? Or is it still valid, meaning you are with the 2 year mark?

If it's the latter, American flyers offers an online FIRC that you can take at your own pace. Once completed, you will be issued a certificate that you can take to your local FSDO, who will then issue you a renewal CFI.

If you need to reinstate you're CFI (meaning it has expired), then you will need to take a checkride.