Place ham in roasting pan and cover with foil. Bake 2-2 1/2 hours
In a small saucepan on medium low. Add the sugar, mustard, root beer, vinegar and cornstarch whisk until combined. Reduce heat to low, and simmer uncovered until reduced and thickened into a glaze, about 20-30 minutes
After about 1 1/2 hours of baking time remove the ham and brush on some glaze. Place ham back into the oven uncovered. Repeat glazing every 15-20 minutes until ham is done.