Best way to boost battery life on my old Android phone?

My trusty Android phone has been slowing down and the battery just doesn’t hold a charge like it used to. I’m not ready to upgrade yet since it still works fine for most stuff, but losing half my battery by midday is a pain. I’ve tried turning off background apps and dimming the screen, but it only helps a little. Anyone else found some tricks or apps that really stretch out battery life on older phones? Would love some easy fixes or settings tweaks before I consider buying a new battery or phone. Also, has anyone tried replacing the battery themselves? How tricky was it?