Select the cell or cells with the drop-down list
Go to the Data tab
Click Data Validation
In the Settings tab, change the source list or criteria
If the list comes from a cell range, update the source range
If the list is typed directly, edit the items in the Source box
Click OK
To change the drop-down options, edit the values in the source cells
To remove the drop-down, open Data Validation and click Clear All or change the Allow setting
To apply the same drop-down to other cells, copy and paste the validated cell or use Fill Down/Fill Right