You can change the PIN or Access Key from the Settings section of the Bitnovo app.
The security PIN or Access Key, which is created with the Bitnovo app, does not serve to back up or restore the wallet, it only has a privacy function to allow access to the application.
IMPORTANT: In case you do not remember the PIN and you need to generate a new one, uninstalling and reinstalling the Bitnovo app, make sure BEFORE you have at hand the 12/24 words of the seed or security phrase that were generated when creating the wallet. You will need the seed to restore your wallet with your balance and transactions.