Like many who haven't done skilled stuff before the OP tosses about words like "easy" so easily. 1

A few points to removing brazed parts from each other. You have to heat the entire brazed area ABOVE the temp it took to join them initially. Single point heat sources, like a torch's flame, is not a good choice for this (see dsaul's reply). If the OEM filler was a brass (as most every production frame has) then this temp is very close to that which will cause localized failure in a thin steel part (think lug or tube).

Next is whether the joint has some other joining method used. Like pins (unlikely in a production bike) or tack welds (more common on 1970s Asian low cost bikes). One needs to know what they are fighting before going into battle.

Of course knowing how the parts are to separate and the direction they will take once loose is important, You don't want to have a hot something falling onto a fuel hose. Since only one joint is reasonable to separate at a time, if the part is attached at more then one end (like a head tube) then cutting it in half and removing one end at a time is the trick.

If you must try to remove the head tube I suggest following dsaul's suggestion of grinding out the old head tube. It's slow but slow brings along an ability to control and limit damage not intended. It's what I've done a few times when replacing a tube. If you're going to shorten the DT then following Eric's idea of just cutting off the old ends/head tube and remitering to install a new HT is far faster. Either way some bending of the top or down tubes to fit the now different geometry goals is likely. Depending on how much bending and what the tubes are this might go pretty straight forward, or not. Then there's the issue of alignment of the new HT WRT the seat tube, the steering angle wanted and also maintaining the BB height/seat tube angle. There's a bunch of things going on all at the same time to consider.

Have you ever brazed? Have you ever pulled apart anything that was brazed? Do you have a practice frame to F up doing your practice? Andy