Select the cell or range where you want the drop-down list
Go to the ribbon: Data → Data Validation
In Allow, select List
In Source, enter one of the following:
Type comma-separated values (e.g., Yes,No,Maybe)
Select a list range (e.g., =A2:A10)
Check the box for In-cell dropdown (if shown)
(Optional) Check Ignore blank if you want blanks allowed
(Optional) Check Show input message (to display a prompt) and/or Show error alert (to restrict invalid entries)
Click OK
To edit the drop-down list: repeat Data → Data Validation and change the Source
To remove the drop-down: select the cells → Data → Data Validation → Clear All → OK