Select the cells where you want the drop-down list
Go to the Data tab
Click Data Validation
In the Allow box, choose List
In the Source box, type the items separated by commas, or select the range containing the list items
Make sure the In-cell dropdown option is checked
Click OK
To edit the list later, return to Data Validation and change the Source
To remove the drop-down, open Data Validation and click Clear All or change the validation settings