To add credits, go to “Accounting” → “Transactions“ in the left-side menu of your dashboard. Click the "Money In" button and select the "Credits" option from the drop-down list:
Enter the date, tenant, sub-category (if needed), method of payment, amount, and press the "Create" button:
Make sure you have added a tenant first so you will be able to give them credits. Later you'll be able to apply the credits toward unpaid invoices.