I pressed receive payment by mistake - i havent received it and its not letting me change back to pending payment. what to do?

Thanks for joining the Community, @rashivijan-gmail. I'm happy to help you unlink the payment to an invoice to make it correct. 

First off, we'll need to open the paid invoice. Then, follow these easy steps to unlink the payment. Here's How: 

1. Go to the Sales menu, then select All Sales.
2. Use the Filter to locate the paid invoice.
3. Under Type, select Invoices, then choose Paid under the Status drop-down. 
4. Click the invoice to open it. 
5. In the Invoice window, click the blue link that says: 1 payment made (amount). Then, click the date to open the Receive Payment page. 
6. Remove the checkmark on the invoice to detach the payment.
7. Hit Set as a credit when a pop-up window prompts.

Your invoice will now back to Open status. while the payment will be saved as a credit. 

For future reference, you'll want to use and apply the credit to an invoice when your customer finally pays it. 

It would always be my pleasure to answer your next posts, @rashivijan-gmail. Have a great day ahead.