We have a gas water heater that we want to replace with an electric one. Is that something that can be done ourselves or do we have to hire someone?

Greetings all, have a small question concerning cold air returns. In my house have one main cold air return near the front door in the foyer. I am certain that the builders put in the cheapest crappiest unit they could find so it takes for ever to cool or heat the house. what I am wondering is if I can put in a few more air returns in the house to make the central unit more effecient. If it will help heat or cool the air faster by pulling from each room, would I be better off putting the returns in the floor, or in the ceiling? and where should I put them with regaurds to the existing vents? near, or other side of the room??? thanks brian