When you enable the Pay Later option in the registration form, all attendees registered using the payment method (Pay Later) will have a Pending payment status
To change the payment status if you have received the payment, you can follow this steps:
1. Once you have logged in to you account, click on your event
2. Go to Management >> Attendees
3. Use the search bar to search for the Attendees name and click on the 3 dots that available at the end of the row
4. Choose View Ticket and the ticket details will appear. Next click the 3 dots on the Ticket Details dialogue
5. From the 3 dots, select Paid
Once you switch the payment to Paid, you will have to send out the confirmation email (for Paid participant) to the participant. To find out how to do it, you may read this article: How to resend confirmation email to attendees?