How to warm up canned beans?
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.