How to warm up canned beans? A recipe is asking for "canned black beans, drained, rinsed and warmed". 

I've never just heated up beans by themselves. How do I warm up these beans without using a microwave? Do I heat them up in a pan with some butter/oil or in simmering water?
I'd use a microwave but you've ruled that out. Taking "warmed" literally as meaning made warm but not hot, you could put them in a bowl, cover with boiling water and wait a few minutes. I suspect that you actually want them hot, in which case you could put them in a small pan, pour over boiling water, cover, and return to the boil for a few minutes. If you don't have an electric kettle, just boil a small pan of water and tip the beans in.