The principle of a kerosene heater is simple: a wick immersed in the fuel becomes saturated, and once lit, the wick will keep burning as long as it remains in contact with the fuel. Wicks need occasional cleaning, because they naturally get clogged with carbon deposits that prevent the fuel from migrating upward. The procedures for cleaning cotton and fiberglass wicks differ.
Turn off the heater and allow 30 minutes for it to cool down.
Raise the wick as far as it will go and clean off carbon deposits with a comb or brush, then retract the wick to make sure no deposits are left to interfere with its movement.
Trim the wick with a pair of scissors if the top edge is ragged and burnt. If you have to trim more than an inch, replace the wick.
Turn the heater off and let it cool for 30 minutes. Brush off any carbon deposits that prevent the wick from retracting, using a comb or brush. If the fiberglass wick is saturated with carbon deposits, you need to burn the wick dry to remove them.
Remove the fuel from the tank to avoid burning it all needlessly before you burn the wick dry. If you can, lift the heater, pour the kerosene into an approved container using a funnel. If the heater is too big to turn over or it's a convection heater, siphon the fuel into the container with a siphon hose. You can buy one at any hardware store.
Take the heater outside and raise the wick as high as it will go. If the unit has a catalytic converter, make sure it's in place. Light the wick and let it burn until it goes out. It will first burn all the kerosene that's in it, then it will burn off the carbon deposits. When the flame goes out, the wick is clean.
Fill the heater with clean fuel, bring it indoors and relight it.