First of all make sure that the power to the geyser is off, test to make sure.
Drain The Water From the Geyser / Water Heater, make sure that the tap supplying the inlet pipe is turned off, open the valve on the drain cock slowly making sure that the water is falling into the drip tray or on the floor, open the valve as much as is needed or all the way if possible.

This will take anywhere between 30 to 60 minutes to drain. then before you remove the power cable to the thermostat, thermal cutout and or to the element take a note of which cable goes where or a photo with your phone so you dont connect the wires wrongly while fitting it back

in some geysers the thermostat will sit next to the element and in others the thermostat will be part of the element. in the latter you will need to remove the thermostat by simply pulling it away from the element.

you will now need a geyser spanner or a cutting player to be able to remove the element, in some instances you will need quite a lot of leverage as the elements do tend to sit pretty tight. when the element is loose and out make sure that the rubber seal came off properly as well. you'll need to clean it thouroughly before inserting the new element with its new rubber seal,

screw it back into place, ensring that you don't cross the thread as you're doing this and fasten properly, insert the thermostat again and reconnect the power cables. do not switch on the power before the geyser is full otherwise you will burn the element out. turn on the water tap to the geyser and open all the hot water taps in your house. the geyser will be properly filled when the water is flowing from all the taps without any air bubbles. you may now switch on the power.