How to repair the battery MacBook Pro mid 2012

to "fix" an 11 year old battery you would need to replace the battery. I would recommend going to OWC’s website to shop for a battery for your specific MBP. and they also have DIY install videos on their site. you can view them to see if it is an operation that you are capable of doing, or if you will need to "farm it out" to a professional.

I would suggest that you see if you may have a Apple Authorized Repair Shop in your area, I have several older out of warranty machines that I have taken to our local Apple Authorized Repair Shop with very good luck.

OWC is also a very good source to use, just remember the battery replacement may not be a task you want to tackle on your own.