If the sealant in your bathtub has become moldy or is just old and not protecting as well as it should, it may be time to replace it. Even if the process seems straight forward enough, check out this website before you start the project.
You’ll find tips for removing old sealant and how to best get it off your hands afterwards.
They suggest, for example, that you fill the bathtub with water before sealing because the tub expands when you do so. If you seal it while empty, it could cause the silicone to shift and become less effective later.