Select the cell or cells with the drop-down list
Go to the Data tab
Click Data Validation
In the Settings tab, check the Source box
Edit the list items directly if they are typed in the Source field
If the Source points to a range, edit the values in that source range
If the Source uses a named range, update the named range in Name Manager
Click OK to save the changes
Copy the updated validation to other cells if needed
Remove and recreate the drop-down if the source cannot be edited directly