I made a roast in the crock pot last night, how long should i heat it up tonight before we eat it?

Does it taste better if i put it back on the crock and heat it up? or should i just microwave it. I put in the fridge this morning before i left for work, and it was perfect. Will it be just as good if i put it on the keep warm setting after work? I have never used a crock pot before, im 19 and learning how to cook. thanks!