If it's expired (beyond 24 months) you will need to retake a checkride for any rating on your instructor cert...the CFII is usually going to be the easiest.
If it is still current, you can either attend a 2 day (16 hour siminar) or take a 16-hour online class. AF and AOPA offer both, most airline folks opt for the online deal because you can do it in a hotel.