I would need more information. Some heaters only have one element others have a top and bottom element. You need to know which element is burned out.That has to be done with with an eletrical tester . I hope you have already checked your breaker or your fuses. some systems still use fuses. Sometimes the metal contact at the back of a glass fuse pushes in, rare but does happen. Easier to change a fuse or reset a breaker.
Water heaters generally have metal panels covering the element acess.held in place by screws. Three important things.TURN OFF THE WATER SUPPLY TO THE HEATER, TURN OFF THE POWER TO THE HEATER and you have to DRAIN THE WATER out of the heater.; There is usually a tap at the bottom of the heater that you can hook a hose onto to drain it. Get the make and model # of the heater and make sure you can get and have the new element ready to go in.
Once you have the acess panel off move the insulation aside and you should see a thermostat and two other bolts or screw that hold the element in.
(If by this time you are still not sure get somebody else to do it) .