How do you change a lower heating element in an electric water heater?
Changing an element us not difficult. Turn the power off.
Turn the water to the water heater off.
Drain the tank. Some models require a special wrench some a crescent wrench will work.
Unscrew the conductors that attach to the element. Unscrew the element.
If it comes with an O ring a very light coating of vegetable oil on the O ring will help it seal. If it uses plumbers dope or PFTE tape coat the threads and install.
Turn water on and fill tank (open  a hot water tap until the air is purged) turn off tap.
Check for leaks. No leaks reattach power leads to element. Turn power on and verify it is still not leaking once warm.