Select the cell with the drop-down list
Go to the Data tab
Click Data Validation
In the Settings tab, edit the Source box
Change the list items or the cell range
Click OK
If the list comes from a named range, go to Formulas
Click Name Manager
Select the named range
Edit the range or formula
Click Close
If the list comes from another sheet, update the source cells directly
If the list is based on a table, add or remove items in the table
Copy the edited validation to other cells if needed