Go to the Invoices list

Select and click the desired invoice

In the bar on the bottom click on the Add Payment

A new window appears

Now you can add/alter the Amount, Date and Note. The default setting is the full invoice amount and today’s date. In case of a partial payment rewrite the sum

Tap on Add

The status changes to Paid or Partially paid and changes color to green

If you preview the invoice, a line saying Paid appears under Total and the Total due sum changes accordingly