If you need to change the balance sheet accounts used in prepayments, open any expense journal page from the Journals tab (click on the box next to the page name).

Use the three dots at the top right of the page to navigate to Expense Journal Settings.

From here, navigate to the expense journals settings for prepayment balance sheet accounts.

See also How to set and change expense journal settings.
For more information about setting up prepayments see Prepaid Expenses: Best Practices and Quick Start Guide
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