Hi Ma101,

Thank you for joining the thread!

Let me share with you the steps on how to mark invoices as paid using the QuickBooks Online mobile app. 

iPhone

1. Select Menu ☰. Then select the All tab.
2. Select Invoice Payments.
3. Select the customer who paid you.
4. Select Edit ✎, then add payment details.
5. When you're done, select Save.

Android

1. Select Menu ☰. Then select the All tab.
2. Select Invoice Payments.
3. Select the Plus + icon.
4. Select Record payment.
5. In the Customer field, select the Plus + icon. Then choose the customer you’re adding a payment for and the customer’s open invoices will display.
6. Tap on the Method field and select the payment method (skip this step if you don’t see the Method field).
7. In the Amount field, enter the amount of the payment you’re receiving.
8. When you're done, select Save.

For your reference, you can also check this article: Record invoice payments in QuickBooks Online. 

Feel free to drop a comment below if you need further assistance with recording an invoice payment.