It's real simple.....go to store...buy new battery.
Bottom line....all those 'tricks' you read about....are just that...tricks....
In some cases, you 'might' get a few extra runs out of a battery...but in about 99% of the batteries, it's done...it's useful life is over....time to replace.
However...there is an exception....in many cases most battery packs are actually a collection of batteries...and in most cases, it's just one or two batteries that are bad. In those cases, you can cut them out...solder in new ones and get some useful life out of the pack.