If the old and new water softeners are similar and if the water connections are similar and accessible, then replacement is usually easy. An experienced plumber should be able to do it in a couple of hours. It may take a homeowner all day. If it's a complicated replacement installation, then it may take a plumber a few hours and a homeowner a day or more. This is probably not a do it yourself project unless you are skilled in dealing with plumbing pipes. You might try our find a professional function to get this job done.