Transfer case drain is a simple drain plug at the lowest point on the case. Fill plug is between case and drum brake. Best removed with a 1/2 inch drive without socket. Make sure you can get the fill plug out before draining lube!
Use a pump devce or plu gun to refill.
Not sure of the spec interval. I do mine annually, probably overkill because of low yearly miles, but it makes me feel better that I see the condition of the lubricant.