In Odoo 16, you can delete or reset a posted expense to draft by following these steps:

1. Go to "Expenses > Expenses > Expenses" and find the expense that you want to delete or reset to draft.

2. Click on the expense to open it in edit mode.

3. If you want to delete the expense, click the "Delete" button at the top of the form. This will delete the expense and all its associated data, including the accounting entries.

4. If you want to reset the expense to draft, click the "Set to Draft" button at the top of the form. This will unpost the expense and reset its status to "Draft".

Note that deleting or resetting a posted expense may have implications for your accounting records, such as reversing the accounting entries that were created when the expense was posted. Make sure to carefully consider the consequences of deleting or resetting a posted expense before taking any action.