how do you lot add drop down list in excel?

Good answer above. One trap to add: pasting over those cells wipes the dropdown silently. Copy something in from another sheet and the validation is just gone, no warning. Paste Special > Values keeps it. Also check the Error Alert tab in the same box. By default it rejects anything not on the list, which is usually what you want, but you can set it to Warning if people need to type the odd one-off.

select the cells first, then Data tab > Data Validation, and set Allow to List. either type the options into Source separated by commas, or click the arrow and select a range of cells that holds them. better habit is putting the options in a column on another sheet and pointing at that, so when they change you edit one place. make that range a proper table and new rows get picked up on their own. and make sure In-cell dropdown is ticked, otherwise you get the validation with no arrow to click.