Depending on the payment processor option you have selected, there may be instances where you'll need to manually apply a payment to a client invoice.
For example, if your client pays you in cash or sends a bank transfer.
Here's how you can Mark the Invoice as Paid (this works for payment installments / partial payments as well!)
Step 1: Navigate to the "Projects" tab on your left side menu + Select the client in question
Step 2: Select the "Invoices" tab (you may have to click "Invoice List" if you have multiple invoices with this client + Select your invoice in question from the list
Step 3: Click "Apply" > "Payment"
Step 4: Input the payment processing date, amount, and method of payment & click "Apply". You should see the payment reflected on the client's invoice total.