iphone4 swollen battery fix it myself?

Hi,

I have an iphone 4 thats now out of warranty. The battery has swollen pushing the top left corner back of the case apart leaving a 3-4 mm gap. The phone still works just fine. The battery swells more when its low on charge, then when I charge it, it shrinks back down almost to the point where it looks normal (still very small gap can't close the case).

Anyone know if Apple will charge just a replacement battery cost to fix this or if they will want to charge me for a whole new phone? how much do they charge for just a battery swap? Should I try buying a replacement battery and fixing it myself?

Please help.

I didn't find the right solution from the Internet.

References:-

https://arstechnica.com/civis/viewtopic.php?f=9&t=1187260

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Thanks!