Repairing a bathtub isn't difficult or time-consuming, and you can easily give your bathtub a new look. The bathroom is a damp room and you have to ventilate it frequently due to the high humidity. Otherwise, mold will start sneaking in, and your efforts to seal and refill will be in vain.
First, clean the bathtub with warm water and soap or an acrylic-specific cleaner. Once dry, use a razor blade or model knife to remove the old filling of the stone resin tub. Once you cut it from end to end, it will come off easily. If some silicone particles don't come off, you can remove them with alcohol or bleach. If the old silicone filler is colored, you may need a commercial filler cleaner to remove all colored parts. You can buy commercial silicone filler cleaners from any store that sells building materials.
You'll need one or two tubes of silicone caulk and a caulk gun, available from any building materials store; you'll also need a model knife or razor, available from some specialty art stores, to remove the old filler. You can use clear or colored fillers. However, if your bathtub has white tiles, it's usually best to use white or colored silicone fillers. Transparent ones are easier to smudge.
You will need to wear rubber gloves as silicone is very sticky and can stick to the skin. It's hard to remove when it dries. Also, if you sprinkle some silicone, it will be easier to clean up with your fingers. That's why gloves are convenient. Also, have a damp towel ready to clean up any excess crevices you accidentally spilled on your tile or colored shower. Pay special attention to these things: they are the hardest part. Trimming at the end is recommended. After you re-caulk with the caulk gun, let it dry for a day or two. At the same time, it is best not to use the bathtub. Once the silicone is dry, reshape with new silicone tubing and let them dry completely before using the stone resin tub.
It doesn't have to be fast, but keep in mind that silicone dries fairly quickly. Be careful not to apply too much silicone around the tile as it will be difficult to clean. Try to avoid inhaling or swallowing the packing, and don't touch your face or eyes while working.