you are also required by the energy efficiency regulations to upgrade your cylinder to the latest high heat recovery coil type when you change it.
as the previous answer said, replacing a cylinder is a big job. you've got to isolate the incoming cold mains supply to the header tank, drain the header tank and cylinder, undo or cut all the pipework connections, get the cylinder out. you will then find the connections on the new cylinder will all be in different places so you are going to have to cut, bend and solder the new pipework and then finally re-fill the system.
best left to a heating engineer/plumber, expect �400 to �500 for the job to be done.