Select the cells where you want the dropdown
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 a cell range containing the items
Make sure the In-cell dropdown option is checked
Click OK
Copy and paste the validated cell to other cells if needed
To edit the dropdown list, change the source items or source range
To remove the dropdown, open Data Validation and click Clear All