It is located on your recovery sheet. On the left of your 24 words you have a QRcode. This QRcode contains the 32 characters of your security card (which you can scan during the update).
On wallets bought after January, the 32 characters are also printed below the QRcode.

If you lost your original recovery card, you can generate a new security card (with the 32 char code) here:
https://fup.hardwarewallet.com/ledger/buildKeycard.html