Do you mean you want to change the list that appears in the dropdown?
If you can find that list, you should be able to edit it.
Is this a combobox on a worksheet?
If yes, is it a dropdown from the Forms toolbar?
If yes, rightclick on it and choose Format control.
then on the Control Tab, look for the Input Range
If it's a combobox from the controltoolbox toolbar
rightclick on it and choose properties (you'll have to show the
controltoolbox toolbar and go into design mode)
Look for the .listfillrange property.
If it's really a Data|validation (looks kind of like a combobox)
select the cell and Data|validation to see the list (and how its defined).
If it's a combobox from that same controltoolbox toolbar, but the list
is built in code, you should find the developer and ask for help.
(If the combobox is on a userform, it's very similar to the combobox from the
controltoolbox toolbar, but look for the .rowsource property (and the same
problem can occur if the list is built by code--see the developer.)