Depends on what you mean by an electric shower. Assuming by pumped you mean there is a separate pump for the shower then new shower:
1. Could be mains fed and 8.5, 9.5, or 10.5 kW, in which case you need to cap off the existing hot and cold feeds as close to their respective cylinder / cistern, disconnect and remove the pump, then run a dedicated feed from the cold mains to the new shower.
2. Could be a power shower with hot and cold feeds and a pump inside the body of the shower. In this case you need to disconnect and remove the existing pump, and run the hot and cold feeds from the point of disconnection to the new shower. Assumes the old pump was properly installed in the first place. Should have isolating valves in both hot and cold feeds.
Too many variables and insufficient definition of the problem to provide a definitive answer.