I boiled milk in my kettle? How do I clean it?

Some water and some bleach.

Alternately, if you have one of those Mr. Clean Magic Erasers, or something similar.

Fill it with water, add some vinegar, heat to boiling, turn burner off and pour hot water out.

Use vinegar.

add some vinegar to the kettle water and boil ..do that a few times ..it may help

It seems to me that the residue of milk inside the kettle can be cleaned by means of sand or gravel or grain;and all you have to do is pouring either one of them into the kettle while water is boiling. By doing that, the boiling water will eventually rub that off.
Yip

Let cool. Take outside. Set over ant hill. Clean in two hours.

Boiling raw milk kills microbes and makes the milk safe to drink. Pasteurized milk is safe to drink cold, but boiling may extend its shelf life. If you just need to heat milk for cooking or to enjoy a warm cup, scalding it is faster and easier
Check whether the milk needs to be boiled. Some milk is safe to drink without boiling. Follow this guide when deciding whether to boil milk:

Raw milk should be boiled whenever possible.
Pasteurized milk should be boiled if it has been stored at room temperature. It does not need to be boiled if it's been in the refrigerator or a very cold room.
A sealed tetra pack with "UHT" on the label is safe to drink, even if it's been stored at room temperature. UHT stand for "ultra high temperature," a type of processing which kills all harmful microbes

vinegar

fill it with water let it sit for 15 minutes then pour soap in it and scrub.

Buy a new kettle