How do I get rid of a brown stain in a electric kettle?

Hydrogen peroxide and baking soda. Make a paste and scrub.

I've been successful with filling it with coffee pot cleaner.

It's from the heating element and the mineral deposits in water. Put some white vinegar in it and turn it on,let it boil then wipe it out may need to scrub with scrubber pad a little or just keep repeating the boiling vinegar thing. Do this regularly and you won't get this build up . This works great in coffee pots put in place of water run thru several times,irons,even dishwashers & clothes washers. Vinegar is a natural water softener. Food safe & makes glass shiny.Reuse & dump hot vinegar in drains,garbage disposal,toilet or on some weeds you've been trying to get rid of.