heeelllooo davie60 and welcome

assuming this is a normal emersion in a central heating system

you need to electricly isolate the emersion
open a tap below the level you are trying to drain to untill you have lowerd the water level below the element

start by tightening the element a fraction if possible to break the bond

make shure the element is exactly the same size[capacity] and no bigger electrical wise

be carefull when extracting as excessive force can dammage the tank or connecting popework

open a hot tap below the level you need to drain to

If there is no draindown on the cylinder (it's usually at the bottom of the cold feed pipe to the cylinder), the only way to get the water out is to loosen the top flange nut (use plenty of towels as there will be some spillage), carefully remove the pipe connection then shove a hose in there and syphon it out into the downstairs sink, or wherever is easiest, to a point below the heater element. If the element is right at the top, there should only a be a few pints of water above it anyway.

As mentioned above, be careful when loosening the heater element. Best to do it with a full cylinder as the water inside gives the walls more strength. Once loosened, you can extract it easier after draining.