If you need to change the balance sheet accounts selected as prepayment for purposes of the Cost Prepayments tab, go to Costs, the three dots at the top right, and select Prepayment Settings.
This will bring you to the Prepayment Settings screen where you can select balance sheet accounts and the threshold for prepayments.
For more about setting up prepayments, see this article.
You can also go to Settings> Organisation settings > Prepayment accounts, and select the relevant accounts.