Is there no way to remove everything so I can put in sand ?
Yes, this is without question the better method. But you may need to get yourself some containers. It is always advisable to have a small tank on hand for emergencies, and one would work here. You don't mention the size of the present tank,but a 10g or a 20 gallon tank might be suitable, depending. A container like a Rubbermaid with a lid, they come in different sizes, can work too. These are also handy for placing hardscape like wood, rock, etc, that you want to keep wet (in tank water, to preserve bacteria on it).
You also need more than one bucket...one in which to rinse the new sand, and one in which to scoop out the existing substrate. If you can do the latter and then dump the old substrate somewhere, it would free the bucket up for rinsing the new sand. I have a set of buckets, handy at times, and well worth it.
Once you have the spare tank/container placed where it will be OK for the day, or overnight, fill it with water from the existing tank, siphoned out from close to the surface (keeps it cleaner). Some of the decor can be moved over, and plants. Then net over the fish. The filter/heater can usually be running in the temporary tank/container.
Then drain the old tank, remove the substrate, rinse it out. Rinse the sand and add it, then the hardscape, some fresh water (use dechlorinator), plants, then either fill with fresh water (at the same temp as the temporary tank water), or drain if the planting made the water a mess, and carefully fill with fresh water. Run it into a large clean bowl to avoid disturbing the sand.
Move the heater and filter over, and if all is well, net the fish over.
