How not to lose existing shelters when reinstalling the game? (Fallout Shelter)

I have all the memory on my iPad, and I don't even understand what. It seems that there are not so many applications and there is no place. In general, I want to reset to factory settings, but then the game will also be deleted. So how can you restore your hideout after installing the game? I have 104 residents there, I've been playing for a long time, it's a pity to lose everything.