Can someone help me repair this battery?

Almost certainly the battery pack is encased in a plastic housing which is sealed and almost impossible to open. You might squeeze it in a vise in hopes of cracking the seal, or you might saw through the plastic. Anything you try risks cutting into the batteries. Better be careful.

Getting at each cell's electrical contacts is a further hurdle. No doubt there's a lot of cells. Test each one at a time with a plain voltmeter. Hope to find any ailing cells. To be fair you should try hooking up a couple volts to charge each one at a time. Then it's a further hurdle to remove ailing cells. Avoid overheating cells via soldering iron. Are the batteries the type that are prone to explode?

Even if you find ailing cells, there may be others waiting to go bad soon. You may decide to make your life easier by purchasing a new battery pack.

Re-assembling the battery pack won't be easy. Nor will sealing everything up again.