To warm up sauerkraut you can put the sauerkraut into a crockpot or slow cooker on the high setting and allow the sauerkraut to warm up for a few hours.
Then keep the crock pot or slow cooker on the low heat setting to keep the sauerkraut warm.
You can also put the sauerkraut into a sauce pan and simmer the sauerkraut on the low setting on the stove for a few minutes.
Just make sure to stir the sauerkraut so that it does not stick to the sauce pan or burn.
Or if you want to you can put the sauerkraut in a microwave safe bowl and heat the sauerkraut up in the microwave for a few minutes.
I sometimes will put a little bit of sauerkraut in the microwave to heat it up if I only need a little bit of sauerkraut.
But if I need more than a serving of Sauerkraut I will use my crock pot which is also a slow cooker to allow the sauerkraut to heat up slowly and stay warm while people are serving themselves with it.