How do you remind a demented individual or make her understand the reasons for hygiene and safety precations?

You can't. Dementia is a horrible illness which slowly (or rapidly) removes the victims ability to understand and communicate. The best you can do is be supportive.
Avoid judgement. Try to avoid asking the person with dementia why they didn't do something, instead focus on getting the task done. If they haven't taken a shower don't ask why or try to explain the importance just say ok it's time to take a shower. Of course this doesn't always work but may help to build habits if you are consistent.