Anywhere in VSys where you credit a volunteer with hours, if that volunteer has any mandates, you'll have the option to associate those hours with a mandate. Note that you can only credit an hours record to one mandate - there's no option to split them across multiple mandates or credit two or more mandates with the same hours. If you need to do either of these things, you can enter the hours in more than one record, each time crediting a different mandate.
For hours which have already been posted, right-clicking on the hours record lets you change which mandate the hours are credited to.