The best and safest way to defrost frozen corned beef is to move it from the freezer into your refrigerator, allowing it to thaw slowly over time. It's important not to leave a frozen item out on the countertop for too long, as this can lead to bacteria growth and unsafe conditions. Generally, depending on the size of your corned beef, you should allow at least 24-48 hours for it to thaw completely in the refrigerator.
Once your corned beef has thawed sufficiently, it needs to be cooked immediately; do not refreeze or store it at room temperature. To cook it properly you'll want to preheat your oven (or another heating element) according to package instructions before placing your thawed corned beef in an appropriate baking dish with about 1/4 cup of water or broth in order for steam release as necessary during cooking. Place a lid or aluminum foil lightly over top and bake according to directions until the internal temperature reads 170 degrees Fahrenheit when tested with a meat thermometer inserted into the thickest part of the roast - about 3 hours for 4 pounds of meat.
Once finished cooking let rest 15 minutes prior to serving so that juices set up evenly throughout the roast then carve against the grain into slices as desired before plating and serving! Enjoy!