You should drop them into boiling water for about 2 1/2 minutes. You can use the microwave but it likely will cause hot spots and make the meat tougher.
To reheat cooked lobster melt butter or olive oil in a fryer, once the butter is hot add the lobster to the pan. Cook/reheat till it is warm to your liking. Avoid using the microwave as it will turn it to rubber.