I have accidentally put an invoice as fully paid instead of partial payment how do I adjust it please?

Let's go to the customer's Transaction List to delete the invoice payment, @kikee. This way, you'll be able to adjust the payment from the full amount to the partial one. I'll guide you how. 

1. Go to Sales from the left menu. 
2. Select Customers.
3. Locate and click the customer's name.
4. In the Transaction List tab, select the paid invoice.
5. Click the 1 payment link beside made on [Date] and choose the date. 
6. In the Receive Payment page, click More. 
7. Choose Delete.
8. Hit Yes to confirm the action. 

The screenshots below show you the last five steps. 

Once done, go back to the Transaction List tab. Then, locate the open invoice and click Receive payment so you can enter its partial payment amount. That way, the system will automatically display the invoice's status as Partial. View the screenshot below for your visual reference. 

Aside from invoices and their payments, you can also manage your other sales transactions in the Transaction List page, such as sales receipts. From there, you can view, edit, copy, delete, void, or print them. To learn more about this process, visit this article: View Sales Transactions. 

I'm here anytime you have other concerns. Enjoy the rest of your day, @kikee.