Upgrading is easy. And there’s no additional cost
For versions 4.0.0.0 and higher:

• Go to ‘Payments’ section of Extensions
• Find PayPal Commerce Platform Integration as recommended option
• Install the module

For versions 3.0.3.8 and higher:

• PayPal Commerce Platform is already available on the list of payment options and doesn’t need to be installed manually.

For versions < 3.0.3.8:

• Download the latest PayPal Commerce Platform module from the OpenCart Marketplace.
• Log in to the OpenCart admin panel. Select Extensions > Installer. Upload the zip file with the payment module under Upload your extensions.
• Go to Extensions > Payments > PayPal Commerce Platform. Select the green button with the plus icon to Install.
• Select Edit to set up payment methods.