According to :
The lower element on my sears water heater is not working -- it looks
as if one of the wires has melted. I'd like to replace it but im
wondering how difficult it is to do. Can anyone let me know?

If one of the supply wires has melted, you may just need to cut
it back to good wire and reattach. Use emory cloth or sandpaper
or something abrasive to clean the terminal contact - a poor
connection is probably why it melted in the first place. Don't
forget to turn the power off first!

If the existing wire becomes too short, grafting on a chunk of
suitably sized wire with wirenuts or replacing it will work.
Get suitably rated wire from an electrical supply store. Eg:
10ga stranded (THHN I think, tell them what it's for. Regular
90C-rated wire should be okay).

Franz gave full and detailed instructions on how to replace the
element. I'd only add: while the element is out of the tank,
inspect the tank through the hole for sediment, and if there is
lots, remove it. If you have a wet/dry shopvac, it'll make it
easy - find a piece of tubing just small enough to go through
the hole, and duct tape it to the shopvac hose. A chunk of garden
hose will work, but you want it as big as possible (sometimes
the sediment is rather, er, "chunky" ;-) yet still fit through
the hole.

Also, i have a upper element part but not a lower. Are these
interchangable or do i need to buy a lower element.

They're often interchangeable. You can check the parts list or
online sources, or compare the one you have with the one you pull out. Or,
perhaps it will have visible part numbers on the outside flange,
so you can check before you have to take the tank out of service
for repair.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.