electric hot water cylinder only gets warm?

If there are two elements (top and bottom) then you are probably correct in assuming the bottom one is kaput. (The alternative is the thermostat is malfunctioning.) Replacing immersion heaters is not easy and you may end up turning the soldered boss into which it screws and causing a leak.(first turn off the water supply and check there is no water coming out of the hot tap)
If there is no drain point on the pipe which goes into the bottom third of the cylinder then you will have to undo the nut at the top of the cylinder and remove the pipe there. Then put a hose pipe into the cylinder right down to the bottom suck on the other end until you get water and drain it into the bath or toilet. Next fit a box spanner (from the merchants) onto the immersion (after turning off the power supply and disconnecting it) and turn gently but firmly anti clockwise to remove. Reverse the procedure to replace an immersion (it should be a 7 inch replacement immersion) If you dont feel confident with what I have descibed call a plumber it should take him an hour or two. Less than 60 pounds parts and labour. Nb. It maybe also be the cylinder is scaled up and may need replacing too.

Bucket head
March 2011