To modify the journal that you've created for your customer, you'll need to look for this entry from the Audit Log section of your account since it's not showing from the customer section. This area keeps track of your activities and allows you to see who made changes to your books and what they did.
To search for the transaction:
Go to Gear icon at the top.
Select on Audit Log.
Select on Filter.
Use the fields on the Filter panel to choose the appropriate User, Date, or Events filter to narrow the results.
Press on Apply.
Once you found the journal entry that you've made, you can now start reversing it. If you want to delete it, just choose Delete from the More section. Please check out the screenshot attached for your reference: