You can access your Instant Account from any web-browser to reset the password.
Instant Account
You can change your Instant account password by clicking here and then on "I've Used Instant Before" and "Forgot Password"
Instant Card in Smartphone Wallet
It is suggested you login to your phone's cloud account to remove any cards. If you do this you can still use the physical Instant Card to access your funds.
You can login to your Instant account via a web-browser and block your card under the Help menu. Doing this will prevent you from using the physical card as well.
More information can be found in this FAQ - My Instant Card is lost or stolen