The Time turners in Harry Potter help them to achieve the events that have ALREADY ACTUALLY HAPPENED… if somebody dies, they cannot be saved! Here’s why:
When they encounter the werewolf Lupin the first time it gets distracted by a wolf call, then when harry and Hermione return the second time it’s Hermoine that makes that wolf call - and so therefore she had made it the first time.
When Harry produces the Stag patronus the second time around he is only able to, because “I already had” - the first time around.
“Future them” are present the first time around but “past them” (and the audience) are unaware of that fact. There are several instances of this the first time around if you watch (or read) it again: Distracting themselves with a pebble, saving buckbeak and pissing off the executioner, making the wolf call, and saving harry and sirius at the lake’s edge. They were there the first time around in all cases so all the things they did when they went back in time HAD ALREADY HAPPENED. They didn’t change anything, they just fulfilled their roll in the events that had already occurred. If future Hermione hadn’t made that wolf call the werewolf wouldn’t have been distracted the first time.
So if something happens, it STAYS happened: they can’t go back to save anyone who dies, because if they do they already would have, and that person wouldn’t be dead. This is a time paradox that JK very cleverly avoided, but maybe wasn’t explained so well in the movies