It is currently not possible to upgrade YubiKey firmware after manufacturing and deployment. To prevent attacks on the YubiKey which might compromise its security, the YubiKey does not permit its firmware to be accessed or altered.
Yubico is dedicated to providing a long-term two-factor authentication solution, we want your YubiKey to remain useful for the full extent of its lifetime. When we do release new firmware, we strive to ensure new YubiKey software will be backwards compatible.