1. From the management settings for an object, go to Search Layouts.
2. For the Lookup Filter Fields layout, click Edit.
3. Use the arrows to add or remove fields from the layout and to define the order in which the fields display. You can add up to six filter fields to the Selected Fields list. To select more than one field, use CTRL+click, or SHIFT+click to select multiple items in a range.
4. Click Save.