If the list of entries ends up being longer or shorter than the original range, go back to the Settings tab and delete what's in the Source box. In this example, you'd add or remove entries in cells A2 through A5. If the Source box contains a reference to a range of cells (for example, =$A$2:$A$5), click Cancel, and then add or remove entries from those cells. For example: Fruits,Vegetables,Meat,Deli. When you're done, each entry should be separated by a comma, with no spaces. If the Source box contains drop-down entries separated by commas, then type new entries or remove ones you don't need. On the Settings tab, click in the Source box.

Select the cells that have the drop-down list.

In Excel for the web, you can only edit a drop-down list where the source data has been entered manually.