On the couple that I replaced I was able to take apart the lower handle assembly (where the cord exits) and wire in a new one where its connected in the handle next to the heater box.
I normally clean out the tanks with hot soapy water and rinse well, alot of tight areas, soaking them for awhile helps if they're really bad. Check for obstructions at the brushroll, check the bladder tank for leaks.
Main thing usually clogged are the spray jets in front (in front of the tank) and on the hand tool, a pin or fine needle works well for clearing out the fine spray hole. The two jets in the base are easily disconnected from the lines by squeezing the clamps.
Running clean water through it with the heat on helps to flush the soap out of the lines.