Go to Accounting > Securities > Edit or delete existing valuation -or- Reports > Valuation > Edit/Delete an existing valuation
Pick the valuation from for the date in question. Click Delete, and then Delete again to confirm. -NOTE: The system will give a warning that removing the valuation will change the unit amounts in member transactions after that date. This is ok.
If you are removing an extra valuation, then you can stop here. If you are taking steps to re-do a valuation, then return to Accounting > Securities, but this time click 'Create new valuation'
Choose the date, and complete the valuation. Since that date has already passed, the system will automatically use the closing prices for that day