OPTION ONE:

If your Vendor Bill journal is setup to Lock Posted Entries with Hash:

Visit Accounting --> Journals --> Purchases and locate the Journal Item representing the Expense debit:

Select it and from the Action menu, choose Automatic Entries:

Fill in the Wizard accordingly:

You will end up with a Journal Entry like this:

OPTION TWO:

If your Vendor Bill journal is not setup to Lock Posted Entries with Hash:

Visit Accounting --> Journals --> Purchases and locate the Journal Item representing the Expense debit:

Select it:

Then change the account by selecting the Account field:

OPTION THREE:

If your Vendor Bill journal is not setup to Lock Posted Entries with Hash:

You can click RESET TO DRAFT on the Vendor Bill, then EDIT to correct the Expense account.  Click CONFIRM to post the Bill again.