Well you can do it with the fish in the tank.... but it's kinder to remove the fish to a bucket for the time you are doing it... with an airstone and the heater if possible....cover the bucket so the fish don't get too stressed.
Invest in a dustpan and brush set...drill small holes in the bottom of the dustpan and use this to scoop out the gravel....it works well as it gets in the corners too.
You are going to have to wash your sand really well...fill a bucket about 1/3rd full with sand and add water...stir vigorously..leave to settle for a few minutes...then pour off the water...repeat until water is clear.
To add sand to your tank....turn off your filter...use a cup or jug....lower the jug/cup into the water then slowly tip out the sand over the bottom of the tank.
Turn on filter and add fish heater back to tank
You will have a problem with water quality for a while until the bacteria colonises the sand.....your gravel held millions of bacteria and you will have cut your colony in half by removing it. You can add some handfulls of gravel to a mesh bag and place on the sand....this will help to colonise the sand quicker.
Cut down on feeding the fish for about a month... won't do them any harm and will help with the water quality.
very confused 
