If you are using the PayPal plugin with the Oxwall software, you simply need to enter the license key.
Go to http://www.oxwall.org/store/granted-list/plugin.
Check if the Reset button show up under the license key. If so, click it.
Copy the license key and enter it while updating the plugin.
That's it.
In case you are using this plugin with the SkaDate software - you should contact their support department.