Lookup field values inherit their sort order and filtering from the default view of the lookup list.
If you need to change the sort order of items that appear in a drop-down that is fed from a list in the site you simply need to modify the default view of the SharePoint list containing the lookup values.
You simply need to add a new column to the lookup list and modify the default view in this list as follows:
- Add a number field to the list naming it "SortOrder" (we used the name SortOrder but you can name it as you wish)
- Edit the default view of the list to sort by the "SortOrder" column you created in step 1
- Finally, set the SortOrder value of each item in the list to determine the order they appear in the dropdown.