Select a cell with the data validation. Click on the Data tab, Data Validation. List should be selected and in the source field there is either a list of values separated by commas or there is an equation referring to a list. If and equation, it could refer to a range of cells that contain the values or it could refer to a Defined Name that contains an equation or a range of cells with the values.

If the Source field has a list separated by commas, add the value(s) that you require. If the Source Field has an equation, go to the list referenced by the equation and add the value(s) and then change the cell references in the equation in Data Validation. If the equation refers to a Defined Name, go to the Name Manager, find the name and make the changes to the range in the formula defining the name after adding the value(s) to the list of values for the data validation.