How to change a customers opening balance after entries have been made?

Hello there, @userrlukas. It's nice to hear from you!

You can change your customer's opening balance by modifying the invoice where the open balance is recorded. Let's proceed with the steps below to get this done:
 

1. Go to the Sales menu.
2. Within the Customers tab, find the invoice with the opening balance.
3. Double click the transaction to open the invoice page.
4. Edit or change the amount.
5. Then, Save or Save and close to reflect the changes.

Learn what accounts are impacted when entering a customer Opening Balance by reading through this article: Method for entering outstanding balances for customers.

I'll be around to help if you need additional assistance with the customer's opening balance or anything related to QuickBooks. Take care and have a good one!