Place ham in a roasting pan and cover with foil. Bake 2 hours
In a small saucepan on medium-low. Add the sugar, mustard, root beer, vinegar, and cornstarch and whisk until combined. Reduce heat to low, and simmer uncovered until reduced and thickened into a glaze, about 20-30 minutes
After about 1 hour 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 the ham is done.
Let rest 15 minutes before serving, and pour juices over the ham.
Notes
You want to make sure your ham is already fully cooked.You can use Dr. Pepper instead of Root Beer if desired.You can place the ham on its side or face down, I just think the glaze gets into the ham slices better if it's on its side.If you use a smaller ham the baking time will need to be reduced.