What`s in there now? If Crushed coral you could siphon out 25% section and then get a piece of PVC and slide the new sand down to that section after you rinse it out. about a week later do another 25% section and so on till you replace all the CC.
You can view many of my fish and corals in my photo albums in my profile.
i would use dry aragonite based sand and not "live" sand. it's much cheaper and the dry sand will become live in a short time anyway. as far as what size grains, i like slightly larger than sugar sized grains.