To change an existing known code:
Press the PROG button (All lights will show)
Enter the Existing code (Day & Tamper Lights show)
Press the "8" button (The 4 zone lights will show)
Now enter your new code (Any 4 Digits) the unit beeps twice & the day & tamper lights show again.
Press the RESET button, & the Day light should show.
If memory serves me correctly - the Concept reverts to factory default code on power-down (which would happen in the event of a power cut and a flat battery!)
0123 should be factory code & you will need a new battery.
In this situation
To change code, pres PROG, enter old code (0123, in this case) then press 8, then new 4 digit code, then reset.