Welcome to the QuickBooks family, @KateHes1.
We can help you figure out why it won't let you change the existing account. However, we need more information about what happens during the process in QuickBooks Online.
Can you share with us if the Payment account field is grayed out, or if you see an error message? Any further details you can add will help us determine the cause of the issue.
Ideally, you should be able to make changes to the expense transaction if you have required access to the account. To check your user type, please click the Gear icon in the upper-right corner and select Manage users.
In case there's a need to modify the access, I suggest reaching out to the Master admin user. They can change what a user can do and see within the company.
You can also try following these steps to change the account:
- Sign into your QuickBooks account.
- On the left panel, click Expenses.
- Search for the expense.
- Select View/Edit below the Action column.

- Change the account.
- Hit Save and close.
I recommend the following resource which has video tutorials to help a user modify expenses, as well as other transactions via register:
Additionally, please check out this link for your reference on how to use QuickBooks Online: https://quickbooks.intuit.com/learn-support/help-articles/misc/03/help-articles-us.
Please know that the Community has your back if you need assistance. It’s a place for you to connect with experts and peers. Feel free to post a question, get answers, share tips, ideas, and more.