First, make sure that you’ve rotated the twisting mechanism at the bottom of the aluminium case as far as it can go (it will stop you twisting any further) so that the inner mechanism is fully wound down. If you're unable to do this step and your old refill is at the top of the case still, apply a bit of pressure to the top of the refill whilst twisting the base, and this should help put the refill into motion again.
Push in the buttons either side of the case and pull the bottom of the case downwards. Get the new refill ready to insert into the case by removing the lid but do not remove the packaging around the refill. Take out the old refill case, put the new refill into the case and reinsert the bottom of the case. Twist the bottom of the case to push the deodorant upwards and apply to your body. We have a handy video for you to watch here too!