Here is the quickest way to bring a tenant back up to date.
Quick method, from the tasks
In the left-hand menu, click Tasks.
Find the line of the relevant delay (Late section), next to the contract you want.
To the right of that line, next to Send a reminder, click the ⋮, then Payment received.
The Add a payment window opens. The Amount of the delay is pre-filled; adjust it if needed.
Check the Date of the payment.
Select the Payment method (Payment on bank account by default).
Add a Description if needed (optional).
Click Save.
The payment is recorded and the delay disappears from your tasks.
Other method: you can also add the payment from the contract's Financials tab (+ Add a transaction → Payment).