The balance in the current asset account Prepaid Expenses should be adjusted prior to issuing a company's financial statements. If the company issues financial statements for each calendar month, you will need to adjust the balance in Prepaid Expenses as of the end of each month. If your company issues only quarterly financial statements, you will need to adjust the balance at the end of each quarter.
The goal is to have the balance in Prepaid Expenses be equal to the amount of the unexpired costs as of the end of the accounting period (which is also the date appearing in the heading of the balance sheet).
Usually the adjusting entry for prepaid expenses will be a credit to Prepaid Expenses and a debit to the appropriate expense account(s). For instance, if Prepaid Expenses involve the prepayment of insurance premiums the adjusting entry will include a debit to Insurance Expense.