Watch the youtube video on battery replacement, if that's for you then go on EPay and buy the battery that comes with the replacement kit/little tools and stuff,
and then don't screw it up.

That's how I did it back when I used that old clunker everyday, 4 devices ago or whatever it was. Still working today as a backup. A very slow backup.

All the different brands of batteries probably come from the same Foxconn factory in China with the suicide nets, just like the phones themselves.