I cannot seem to edit an existing invoice. Client paid partialy and decided not to take the one product, so I want to change the qty sold

Hello, userinfo8.

I'd be glad to lend you a hand and fix the invoice problem so you can edit it. 

Did you encounter an error message? If so, include it in your reply below. That way, I can look for suitable troubleshooting steps that can resolve the problem

In the meantime, you can do these steps to fix the issue. A browser cache problem can prevent you from editing an existing invoice. A full or corrupted cache storage usually causes unexpected behavior in the webpage. To confirm this, let's use an incognito or private browser: 

• Chrome, press CTRL + Shift + N
• Mozilla Firefox: press CTRL + Shift + P
• Safari (newer versions): press Command + Shift + N

Next, log in to https://qbo.intuit.com/ from there and edit your invoice. If you're able to do so, let's close out the private window and go back to the regular browser. Then, clear the cache to fix the issue. Finally, close out the browser to complete the process. 

If you're using the QuickBooks Online Windows app, reset the data to fix this (works similarly to clearing the cache). Simply click the Help menu, then select Reset App Data. 

If you need help editing your invoice after doing the steps, I'll show you how: 

1. Open the invoice.
2. Change the amount under QTY. 
3. If you want to remove an item, simply click the trash can icon.
4. Once done, click Save or Save and close.

Next, record the payment by clicking the Receive payment button (on the invoice screen). You can find more details about recording invoice payments here. 

Looking to personalise your invoice and other sales forms? You can check out this article for the steps and details how: Customise invoices, quotes, and sales receipts in QuickBooks Online.

I'm still here if you need help with other things in QuickBooks Online. Post a reply down below and I'll get back to you as soon as possible.