Hotend disassembly & heatbreak stuck in the heatsink

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You can find the complete E3Ds assembly manual of the V6 hotend, which can be followed backward for disassembly. This is the stock hotend on an Original Prusa MK3S/MK2.5S/MK2S. However, as the manual works with brand new parts, it does not consider the changes that happen when being heated repeatedly to hundreds of degrees, again and again. Metal contracts and expands and thermal paste will turn crusty over time. Therefore there are some changes to the procedure when disassembling after a long time of use. 

Disassembly

The assembly manual for the V6 is still optimal for most of this procedure, with a few exceptions. Moving backward, you are first removing the nozzle (at 280 °C) to release the heatbreak. However, this makes it harder to remove the heatbreak from the heat sink in later steps.

As the heatbreak can be quite fragile, and its threads me be damaged if you try to grip it with pliers, we recommend that you unscrew the heatsink from the heatbreak, before removing the nozzle, which locks the heatbreak in place. A Heatbreak can still bend and break so do not unscrew it at an angle. A bend will create a whole range of printing issues.

After you have unscrewed the heat sink from the heatbreak, proceed with the E3Ds assembly manual, by heating the block and unscrewing the nozzle.

Too late, it is already stuck!

Follow this procedure to resolve the issue.

1. Without any heat, manually screw your nozzle into its threaded hole in the heater block. 
2. Screw it all the way in, then loosen it back a full turn (as described in step 3 of the E3D V6 Hotend assembly) creating a gap of about 1mm between nozzle and block.
3. Then screw the heatbreak, with the attached heatsink, into the heatblock from the other side as far as it will go.
4. Tighten the nozzle with a pair of pliers and you now have a better handle. You can also grab this with pliers for some leverage when unscrewing the stuck heatsink.