You can recertify at any time before your certification reaches the expired state.
Once your certification is in expired state, you have 6 months to pass a recertification exam to renew your certification before it reaches termination status. If you pass the recertification exam, your certification will remain active, and you will maintain your original certification ID.
Please note, however, that you must recertify prior to your certification termination date, or else it will reach termination status and you will have to restart your certification. We recommend ensuring you recertify at least a week before your termination date.
Please visit the recertification page associated with the certification program you are interested in to find more information on recertification.
SRC: https://www.nokia.com/networks/training/src/recertification/
ONC: https://www.nokia.com/networks/training/onc/recertification/