Download the latest version of the Montonio OpenCart plugin from HERE.

In the OpenCart admin panel, navigate to Extensions > Installer and uninstall the old package (by doing this the Montonio payment methods settings will be deleted, you will have to re-configure Montonio payment methods again after the update).

Click the “Upload” button and select the new “montonio.ocmod” package (the extension will be installed automatically).

Navigate to Extensions > Extensions and select “Payments” in dropdown.

Scroll down to Montonio payment methods, and click the “+” button to install the methods you want to add to your checkout.

Once the method is installed, you can click the button with a pencil icon to configure the payment method.

Set the status to “Enabled” and add API keys (make sure you use the correct environment API keys, LIVE keys are different from sandbox keys). Save the settings.

Order status after successful payment - Make sure you have configured the "order status after successful payment". Otherwise, new orders will be in "canceled" status if you don't configure it.