baked last night refrigatered
A few minutes in stovetop or microwave. directions in the link. They say to use milk or water, but/and I find stock gives more flavor. Your choice.
I am going to provides some ideal process of reheating grits. I find this solution from https://foodgear.org/how-to-reheat-grits/, this site. Anyways, If you have refrigerated them, bring it to the room temperature first. After it comes to normal temperature, you can either use the stovetop or microwave it for heating. If you want to know the step by step procedures you can visit the above-mentioned site link.