From the menu bar (assuming you are not using Office 2007):
The "Allow" box should be set to "List"
The "Source" box will contain one of two things, either the list of valid
values for the drop-down or a reference to the cells elsewhere on the sheet
that contain the valid values. Edit either as appropriate.
Note that if a cell reference is used, the text may have been set to white
and the cells locked to try to hide them from accidential/intentional change.