I think there are instructions in the handbook about reprogramming the key. RTFM

Alternatively - a two second Google search brings up the answer:

FORD Fiesta 96> & Escort 98> (Radio 3 Button) - Key Remote Programming
1. Insert Key and turn to II, then Turn Key from II to I and back to II four times within 3 seconds
2. The PATS LED should light. Turn Key to Position 0.
3. Press the Unlock button and keep it pressed until the PATS LED flashes 4 times.
4. Add additional fobs the same way. Turn to II and back to 0 to end.