If you need to change the balance sheet accounts used in prepayments, go to Journals (top tab)>Prepaid Expenses (any page)>cog (top right corner)>Prepaid expenses settings.

This will bring you to the Prepayment Settings screen where you can select balance sheet accounts and the threshold for prepayments.

For more information about setting up prepayments see Prepaid Expenses:  Best Practices and Quick Start Guide