It's quite easy to change from sand. Just drain most of your water into buckets or some other safe container(s). Keep your fish in there while your doing this. Grab a plastic shovel or something to scoop out the gravel. Once the gravel is all out, go ahead and put in the sand (but rinse it really well first). Once the sand is in it's only a matter of putting everything back in. Make sure your filter intakes arent too close to the substrate because it's possible to get it in your filters and that's not good for them. Keep in mind this causes a biological disturbance and you may get some ammonia or nitrite showing up shortly afterward depending on how your tank is stocked. The key is to do it quick, make sure the sand is washed before you start.